Swift 6.1 OSS toolchain cannot build packages when Xcode 26 is installed

[marcprux]

Description

Anything that imports Foundation cannot be built with and of the Swift 6.1 OSS toolchains when Xcode 26 is installed as the default.

marc@zap SampleProject % swiftly run swift build +6.1
Building for debugging...
error: emit-module command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Modules/Foundation.swiftmodule/arm64e-apple-macos.swiftinterface:6:8: error: failed to build module 'Combine'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
    4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
    5 | import Builtin
    6 | import Combine
      |        `- error: failed to build module 'Combine'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
    7 | import CoreFoundation
    8 | import Darwin

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

<module-includes>:1:9: note: in file included from <module-includes>:1:
1 | #import "Headers/CoreFoundation.h"
  |         `- note: in file included from <module-includes>:1:
2 | #import "Headers/CFPlugInCOM.h"
3 | 

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:10: error: could not build module '_Builtin_float'
 21 | #include <ctype.h>
 22 | #include <errno.h>
 23 | #include <float.h>
    |          `- error: could not build module '_Builtin_float'
 24 | #include <limits.h>
 25 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/CoreFoundation.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Dispatch.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Dispatch.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/System.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/private/tmp/SampleProject/Sources/SampleProject/SampleProject.swift:1:8: error: failed to build module 'Foundation'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
1 | import Foundation
  |        `- error: failed to build module 'Foundation'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
2 | 
3 | func someFoundationFunction() -> URL? {

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:10: note: while building module 'CoreFoundation' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
  4 | 
  5 | 
  6 | #include <CoreFoundation/CoreFoundation.h>
    |          `- note: while building module 'CoreFoundation' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
  7 | 
  8 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:10: note: while building module '_Builtin_float' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:
 21 | #include <ctype.h>
 22 | #include <errno.h>
 23 | #include <float.h>
    |          `- note: while building module '_Builtin_float' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:
 24 | #include <limits.h>
 25 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:1: note: in file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:
 90 | 
 91 | // Base interfaces below the C standard library.
 92 | extern module os_availability "DarwinFoundation1.modulemap"
    | `- note: in file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:
 93 | extern module sys_appleapiopts "DarwinFoundation1.modulemap"
 94 | extern module sys_cdefs "DarwinFoundation1.modulemap"
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Modules/Foundation.swiftmodule/arm64e-apple-macos.swiftinterface:6:8: error: failed to build module 'Combine'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
    4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
    5 | import Builtin
    6 | import Combine
      |        `- error: failed to build module 'Combine'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
    7 | import CoreFoundation
    8 | import Darwin

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

<module-includes>:1:9: note: in file included from <module-includes>:1:
1 | #import "Headers/CoreFoundation.h"
  |         `- note: in file included from <module-includes>:1:
2 | #import "Headers/CFPlugInCOM.h"
3 | 

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:10: error: could not build module '_Builtin_float'
 21 | #include <ctype.h>
 22 | #include <errno.h>
 23 | #include <float.h>
    |          `- error: could not build module '_Builtin_float'
 24 | #include <limits.h>
 25 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/CoreFoundation.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap:306:8: error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
304 | // is here to generate early errors if client search paths are set up
305 | // incorrectly.
306 | module _c_standard_library_obsolete [system] {
    |        `- error: module '_c_standard_library_obsolete' requires feature 'found_incompatible_headers__check_search_paths'
307 |   requires found_incompatible_headers__check_search_paths
308 | 

/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h:32:17: note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 30 | #endif
 31 | 
 32 | #  include_next <float.h>
    |                 `- note: submodule of top-level module '_c_standard_library_obsolete' implicitly imported here
 33 | 
 34 | /* Undefine anything that we'll be redefining below. */

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Dispatch.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Combine.framework/Modules/Combine.swiftmodule/arm64e-apple-macos.swiftinterface:5:8: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios18.0-macabi -enable-objc-interop -enable-library-evolution -swift-version 5 -enforce-exclusivity=checked -O -library-level api -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-library-evolution -enable-experimental-feature DebugDescriptionMacro -enable-bare-slash-regex -user-module-version 3023 -module-name Combine
   4 | // swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
   5 | import Darwin
     |        `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
   6 | import Swift
   7 | import _Concurrency

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Dispatch.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface:1:1: error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  1 | // swift-interface-format-version: 1.0
    | `- error: failed to build module 'Darwin'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
  2 | // swift-compiler-version: Apple Swift version 6.2 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)
  3 | // swift-module-flags: -target arm64e-apple-macos26.0 -target-variant arm64e-apple-ios26.0-macabi -enable-objc-interop -enable-library-evolution -module-link-name swiftDarwin -swift-version 6 -enforce-exclusivity=checked -O -library-level api -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature DebugDescriptionMacro -user-module-version 347.0.12 -module-name Darwin

<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/Darwin.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/swift/System.swiftmodule/arm64e-apple-macos.swiftinterface: 0:
<unknown>:0: error: could not build Objective-C module '_Builtin_float'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/DarwinFoundation1.modulemap: 0:
/Users/marc/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/clang/include/float.h: 0:
/private/tmp/SampleProject/Sources/SampleProject/SampleProject.swift:1:8: error: failed to build module 'Foundation'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
1 | import Foundation
  |        `- error: failed to build module 'Foundation'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 6.2 effective-5.10 (swiftlang-6.2.0.17.14 clang-1700.3.17.1)', while this compiler is 'Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)'). Please select a toolchain which matches the SDK.
2 | 
3 | func someFoundationFunction() -> URL? {

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:10: note: while building module 'CoreFoundation' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
  4 | 
  5 | 
  6 | #include <CoreFoundation/CoreFoundation.h>
    |          `- note: while building module 'CoreFoundation' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
  7 | 
  8 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:10: note: while building module '_Builtin_float' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:
 21 | #include <ctype.h>
 22 | #include <errno.h>
 23 | #include <float.h>
    |          `- note: while building module '_Builtin_float' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:23:
 24 | #include <limits.h>
 25 | #if !0

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:1: note: in file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:
 90 | 
 91 | // Base interfaces below the C standard library.
 92 | extern module os_availability "DarwinFoundation1.modulemap"
    | `- note: in file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/module.modulemap:92:
 93 | extern module sys_appleapiopts "DarwinFoundation1.modulemap"
 94 | extern module sys_cdefs "DarwinFoundation1.modulemap"

The only known workaround is to keep an old version of Xcode on the machine and set it as the default with:

sudo xcode-select --switch /Applications/Xcode-16.4.app

Reproduction

// Sources/SampleProject/SampleProject.swift
import Foundation
func someFoundationFunction() -> Data { Data() }

Expected behavior

Swift 6.1 toolchains should not break just because Xcode is upgraded.

Environment

Xcode 26

marc@zap SampleProject % sw_vers 
ProductName:		macOS
ProductVersion:		15.6.1
BuildVersion:		24G90

marc@zap SampleProject % xcrun --version
xcrun version 72.

marc@zap SampleProject % swiftly list
Installed release toolchains
----------------------------
Swift 6.2.0 (in use) (default)
Swift 6.1.2
Swift 6.1.0

Installed snapshot toolchains
-----------------------------
6.2-snapshot-2025-08-14
6.2-snapshot-2025-08-13
6.2-snapshot-2025-08-09

Additional information

No response

[mihai-chiorean]

+1 running into this too. Also it looks like what I build with 6.2 won't run.

dyld[56231]: Library not loaded: @rpath/libswiftCompatibilitySpan.dylib
  Referenced from: <F03D72C8-B71B-37BA-91ED-CBCC107A8A2F> /Users/mihaichiorean/workspace/edgeengineer/edge-agent/.build/arm64-apple-macosx/debug/edge
  Reason: tried: '/Users/mihaichiorean/workspace/edgeengineer/edge-agent/.build/arm64-apple-macosx/debug/libswiftCompatibilitySpan.dylib' (no such file), '/Users/mihaichiorean/workspace/edgeengineer/edge-agent/.build/arm64-apple-macosx/debug/libswiftCompatibilitySpan.dylib' (no such file)

[sebsto]

I hit this too.
Removing .build and .swiftpm usually solved it

Now, when running swift test I hit the same error as @mihai-chiorean

Library not loaded: @rpath/libswiftCompatibilitySpan.dylib
Referenced from: <677E30C2-1F0C-3301-874D-DEA38DDD99F0> /Users/stormacq/Documents/amazon/code/swift/mac/xcodeinstall/.build/arm64-apple-macosx/debug/xcodeinstallPackageTests.xctest/Contents/MacOS/xcodeinstallPackageTests

[sebsto]

looks like it will be fixed in 6.2.1
swiftlang/swift-package-manager#9163

We can probably close this as this is a duplicate of swiftlang/swift-package-manager#9163