[Dependency Scanning] Warn, instead of fail, when a Swift dependency query only finds modules built for incompatible target

include/swift/Serialization/ScanningLoaders.h

@@ -22,6 +22,11 @@ namespace swift {
 /// Result of looking up a Swift module on the current filesystem
 /// search paths.
 struct SwiftModuleScannerQueryResult {
+  // Checked for by the scanner as a special case
+  // for downgrading imcompatible-candidate-only lookup result
+  // to a warning.
+  static constexpr const char *BUILT_FOR_INCOMPATIBLE_TARGET =
+      "built for incompatible target";
   struct IncompatibleCandidate {
     std::string path;
     std::string incompatibilityReason;

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -2088,6 +2088,19 @@ void ModuleDependencyIssueReporter::diagnoseFailureOnOnlyIncompatibleCandidates(
   if (candidates.empty())
     return;
 
+  // FIXME: There are known cases where clients are relying on
+  // loading the underlying Clang module in the presence of a Swift
+  // module which is lacking the required target-specific variant,
+  // such as MacCatalyst. Eventually, we should pursue making this
+  // an error as well.
+  if (llvm::all_of(candidates, [](auto &incompatibleCandidate) {
+        return incompatibleCandidate.incompatibilityReason ==
+            SwiftModuleScannerQueryResult::BUILT_FOR_INCOMPATIBLE_TARGET;
+      })) {
+    warnOnIncompatibleCandidates(moduleImport.importIdentifier, candidates);
+    return;
+  }
+
   diagnoseModuleNotFoundFailure(moduleImport, cache, dependencyOf,
                                 /* resolvingSerializedSearchPath */ std::nullopt,
                                 candidates);

lib/Serialization/ScanningLoaders.cpp

@@ -117,7 +117,7 @@ bool SwiftModuleScanner::handlePossibleTargetMismatch(
 
   for (const auto &modulePath : foundIncompatibleArchModules)
     incompatibleCandidates.push_back({modulePath,
-                                      "invalid architecture"});
+      SwiftModuleScannerQueryResult::BUILT_FOR_INCOMPATIBLE_TARGET});
 
   return false;
 }

test/ScanDependencies/incompatible-arch-underlying-clang.swift

@@ -0,0 +1,25 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/deps)
+// RUN: %empty-directory(%t/module-cache)
+// RUN: %empty-directory(%t/inputs/Foo.swiftmodule)
+// RUN: touch %t/inputs/Foo.swiftmodule/i387.swiftmodule
+// RUN: touch %t/inputs/Foo.swiftmodule/ppc65.swiftmodule
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/module-cache %t/client.swift -o %t/deps.json -I %t/inputs -I %t/deps -diagnostic-style llvm -scanner-module-validation 2>&1 | %FileCheck %s
+
+// CHECK-DAG: warning: module file '{{.*}}Foo.swiftmodule{{/|\\}}ppc65.swiftmodule' is incompatible with this Swift compiler: built for incompatible target
+// CHECK-DAG: warning: module file '{{.*}}Foo.swiftmodule{{/|\\}}i387.swiftmodule' is incompatible with this Swift compiler: built for incompatible target
+// CHECK-NOT: error
+
+//--- deps/foo.h
+void foo(void);
+
+//--- deps/module.modulemap
+module Foo {
+  header "foo.h"
+  export *
+}
+
+//--- client.swift
+import Foo

test/ScanDependencies/incompatible-arch.swift

@@ -6,7 +6,8 @@
 
 // RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/module-cache %s -o %t/deps.json -I %t/inputs -diagnostic-style llvm -scanner-module-validation 2>&1 | %FileCheck %s
 
-// CHECK: error: unable to resolve Swift module dependency to a compatible module: 'Foo'
-// CHECK-DAG: note: found incompatible module '{{.*}}Foo.swiftmodule{{/|\\}}ppc65.swiftmodule': invalid architecture
-// CHECK-DAG: note: found incompatible module '{{.*}}Foo.swiftmodule{{/|\\}}i387.swiftmodule': invalid architecture
+// CHECK-DAG: warning: module file '{{.*}}Foo.swiftmodule{{/|\\}}ppc65.swiftmodule' is incompatible with this Swift compiler: built for incompatible target
+// CHECK-DAG: warning: module file '{{.*}}Foo.swiftmodule{{/|\\}}i387.swiftmodule' is incompatible with this Swift compiler: built for incompatible target
+// CHECK: error: unable to resolve module dependency: 'Foo'
+
 import Foo