diff --git a/cmake/modules/SwiftSource.cmake b/cmake/modules/SwiftSource.cmake
index ac9701205a752..aa525d3c1d697 100644
--- a/cmake/modules/SwiftSource.cmake
+++ b/cmake/modules/SwiftSource.cmake
@@ -59,9 +59,6 @@ function(handle_swift_sources
   if (NOT SWIFTSOURCES_IS_MAIN)
     list(APPEND swift_compile_flags "-module-link-name" "${name}")
   endif()
-  if("${SWIFTSOURCES_SDK}" STREQUAL "CYGWIN")
-    list(APPEND swift_compile_flags -DCYGWIN)
-  endif()
 
   if(swift_sources)
     compute_library_subdir(SWIFTSOURCES_LIBRARY_SUBDIR
diff --git a/lib/Basic/LangOptions.cpp b/lib/Basic/LangOptions.cpp
index d648b4abf6723..6906b011585b3 100644
--- a/lib/Basic/LangOptions.cpp
+++ b/lib/Basic/LangOptions.cpp
@@ -36,6 +36,7 @@ static const StringRef SupportedConditionalCompilationOSs[] = {
   "Windows",
   "Android",
   "PS4",
+  "Cygwin",
 };
 
 static const StringRef SupportedConditionalCompilationArches[] = {
@@ -157,6 +158,8 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
     addPlatformConditionValue(PlatformConditionKind::OS, "FreeBSD");
   else if (triple.isOSWindows())
     addPlatformConditionValue(PlatformConditionKind::OS, "Windows");
+  else if (triple.isWindowsCygwinEnvironment())
+    addPlatformConditionValue(PlatformConditionKind::OS, "Cygwin");
   else if (triple.isPS4())
     addPlatformConditionValue(PlatformConditionKind::OS, "PS4");
   else
diff --git a/stdlib/private/StdlibUnittest/RaceTest.swift b/stdlib/private/StdlibUnittest/RaceTest.swift
index e88288d2786c7..08d1938402e14 100644
--- a/stdlib/private/StdlibUnittest/RaceTest.swift
+++ b/stdlib/private/StdlibUnittest/RaceTest.swift
@@ -41,7 +41,7 @@ import SwiftPrivateLibcExtras
 import SwiftPrivatePthreadExtras
 #if os(OSX) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
diff --git a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
index 7307e81e9d724..960d1ffc9ce27 100644
--- a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
+++ b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
@@ -14,7 +14,7 @@ import SwiftPrivate
 import SwiftPrivateLibcExtras
 #if os(OSX) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
diff --git a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
index d8e130c7b16a7..5f7d2748f268b 100644
--- a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
+++ b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
@@ -20,7 +20,7 @@ import SwiftPrivateLibcExtras
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
@@ -1497,7 +1497,7 @@ func _getOSVersion() -> OSVersion {
   return .ps4
 #elseif os(Android)
   return .android
-#elseif os(Windows) && CYGWIN
+#elseif os(Cygwin)
   return .windowsCygnus
 #elseif os(Windows)
   return .windows
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index 618e24b46a2c6..e4350d92d254d 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -13,13 +13,14 @@
 import SwiftPrivate
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
 
-#if !os(Windows) || CYGWIN
-// posix_spawn is not available on Android or Windows.
+#if !os(Windows)
+// posix_spawn is not available on Windows.
+// posix_spawn is not available on Android.
 #if !os(Android)
 // swift_posix_spawn isn't available in the public watchOS SDK, we sneak by the
 // unavailable attribute declaration here of the APIs that we need.
@@ -264,7 +265,7 @@ public enum ProcessTerminationStatus : CustomStringConvertible {
 
 public func posixWaitpid(_ pid: pid_t) -> ProcessTerminationStatus {
   var status: CInt = 0
-#if CYGWIN
+#if os(Cygwin)
   withUnsafeMutablePointer(to: &status) {
     statusPtr in
     let statusPtrWrapper = __wait_status_ptr_t(__int_ptr: statusPtr)
@@ -304,7 +305,7 @@ internal func _getEnviron() -> UnsafeMutablePointer<UnsafeMutablePointer<CChar>?
   return environ
 #elseif os(Android)
   return environ
-#elseif CYGWIN
+#elseif os(Cygwin)
   return environ
 #else
   return __environ
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index 2850883eff664..8d4412fb5cbb3 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -13,13 +13,13 @@
 import SwiftPrivate
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #elseif os(Windows)
 import ucrt
 #endif
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows)
 public func _stdlib_mkstemps(_ template: inout String, _ suffixlen: CInt) -> CInt {
 #if os(Android)
   preconditionFailure("mkstemps doesn't work on Android")
@@ -85,7 +85,7 @@ public struct _stdlib_fd_set {
   }
 }
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows)
 public func _stdlib_select(
   _ readfds: inout _stdlib_fd_set, _ writefds: inout _stdlib_fd_set,
   _ errorfds: inout _stdlib_fd_set, _ timeout: UnsafeMutablePointer<timeval>?
@@ -99,7 +99,7 @@ public func _stdlib_select(
         let readAddr = readfds.baseAddress
         let writeAddr = writefds.baseAddress
         let errorAddr = errorfds.baseAddress
-#if CYGWIN
+#if os(Cygwin)
         typealias fd_set = _types_fd_set
 #endif
         func asFdSetPtr(
@@ -125,10 +125,10 @@ public func _stdlib_select(
 public func _stdlib_pipe() -> (readEnd: CInt, writeEnd: CInt, error: CInt) {
   var fds: [CInt] = [0, 0]
   let ret = fds.withUnsafeMutableBufferPointer { unsafeFds -> CInt in
-#if !os(Windows) || CYGWIN
-    return pipe(unsafeFds.baseAddress)
-#else
+#if os(Windows)
     return _pipe(unsafeFds.baseAddress, 0, 0)
+#else
+    return pipe(unsafeFds.baseAddress)
 #endif
   }
   return (readEnd: fds[0], writeEnd: fds[1], error: ret)
diff --git a/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift b/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
index 2ebe4b3accaf7..df922f9a8877c 100644
--- a/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
+++ b/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
@@ -12,7 +12,7 @@
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
@@ -43,7 +43,7 @@ public var _stdlib_PTHREAD_BARRIER_SERIAL_THREAD: CInt {
 }
 
 public struct _stdlib_pthread_barrier_t {
-#if CYGWIN || os(FreeBSD)
+#if os(Cygwin) || os(FreeBSD)
   var mutex: UnsafeMutablePointer<pthread_mutex_t?>?
   var cond: UnsafeMutablePointer<pthread_cond_t?>?
 #else
diff --git a/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift b/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
index fe0c4f06586d7..26809a042e90f 100644
--- a/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
+++ b/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
@@ -17,7 +17,7 @@
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 import Glibc
 #endif
 
@@ -59,7 +59,7 @@ internal func invokeBlockContext(
   return context.run()
 }
 
-#if CYGWIN || os(FreeBSD)
+#if os(Cygwin) || os(FreeBSD)
 public typealias _stdlib_pthread_attr_t = UnsafePointer<pthread_attr_t?>
 #else
 public typealias _stdlib_pthread_attr_t = UnsafePointer<pthread_attr_t>
diff --git a/stdlib/public/Platform/Platform.swift b/stdlib/public/Platform/Platform.swift
index 1512aa04bb1f0..fd7c6605e225a 100644
--- a/stdlib/public/Platform/Platform.swift
+++ b/stdlib/public/Platform/Platform.swift
@@ -141,23 +141,23 @@ public func snprintf(ptr: UnsafeMutablePointer<Int8>, _ len: Int, _ format: Unsa
 // fcntl.h
 //===----------------------------------------------------------------------===//
 
-#if !os(Windows) || CYGWIN
+#if os(Windows)
 @_silgen_name("_swift_Platform_open")
 func _swift_Platform_open(
   _ path: UnsafePointer<CChar>,
   _ oflag: Int32,
-  _ mode: mode_t
+  _ mode: Int32
 ) -> Int32
 #else
 @_silgen_name("_swift_Platform_open")
 func _swift_Platform_open(
   _ path: UnsafePointer<CChar>,
   _ oflag: Int32,
-  _ mode: Int32
+  _ mode: mode_t
 ) -> Int32
 #endif
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows)
 @_silgen_name("_swift_Platform_openat")
 func _swift_Platform_openat(
   _ fd: Int32,
@@ -174,7 +174,15 @@ public func open(
   return _swift_Platform_open(path, oflag, 0)
 }
 
-#if !os(Windows) || CYGWIN
+#if os(Windows)
+public func open(
+  _ path: UnsafePointer<CChar>,
+  _ oflag: Int32,
+  _ mode: Int32
+) -> Int32 {
+  return _swift_Platform_open(path, oflag, mode)
+}
+#else
 public func open(
   _ path: UnsafePointer<CChar>,
   _ oflag: Int32,
@@ -199,17 +207,9 @@ public func openat(
 ) -> Int32 {
   return _swift_Platform_openat(fd, path, oflag, mode)
 }
-#else
-public func open(
-  _ path: UnsafePointer<CChar>,
-  _ oflag: Int32,
-  _ mode: Int32
-) -> Int32 {
-  return _swift_Platform_open(path, oflag, mode)
-}
 #endif
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows)
 @_silgen_name("_swift_Platform_fcntl")
 internal func _swift_Platform_fcntl(
   _ fd: Int32,
@@ -248,7 +248,18 @@ public func fcntl(
 }
 #endif
 
-#if !os(Windows) || CYGWIN
+#if os(Windows)
+public var S_IFMT: Int32 { return Int32(0xf000) }
+
+public var S_IFREG: Int32 { return Int32(0x8000) }
+public var S_IFDIR: Int32 { return Int32(0x4000) }
+public var S_IFCHR: Int32 { return Int32(0x2000) }
+public var S_IFIFO: Int32 { return Int32(0x1000) }
+
+public var S_IREAD: Int32  { return Int32(0x0100) }
+public var S_IWRITE: Int32 { return Int32(0x0080) }
+public var S_IEXEC: Int32  { return Int32(0x0040) }
+#else
 public var S_IFMT: mode_t   { return mode_t(0o170000) }
 public var S_IFIFO: mode_t  { return mode_t(0o010000) }
 public var S_IFCHR: mode_t  { return mode_t(0o020000) }
@@ -286,24 +297,13 @@ public var S_IREAD: mode_t  { return S_IRUSR }
 public var S_IWRITE: mode_t { return S_IWUSR }
 public var S_IEXEC: mode_t  { return S_IXUSR }
 #endif
-#else
-public var S_IFMT: Int32 { return Int32(0xf000) }
-
-public var S_IFREG: Int32 { return Int32(0x8000) }
-public var S_IFDIR: Int32 { return Int32(0x4000) }
-public var S_IFCHR: Int32 { return Int32(0x2000) }
-public var S_IFIFO: Int32 { return Int32(0x1000) }
-
-public var S_IREAD: Int32  { return Int32(0x0100) }
-public var S_IWRITE: Int32 { return Int32(0x0080) }
-public var S_IEXEC: Int32  { return Int32(0x0040) }
 #endif
 
 //===----------------------------------------------------------------------===//
 // ioctl.h
 //===----------------------------------------------------------------------===//
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows)
 @_silgen_name("_swift_Platform_ioctl")
 internal func _swift_Platform_ioctl(
   _ fd: CInt,
@@ -380,8 +380,7 @@ public var SIG_ERR: sighandler_t {
 public var SIG_HOLD: sighandler_t {
   return unsafeBitCast(2, to: sighandler_t.self)
 }
-#elseif os(Windows)
-#if CYGWIN
+#elseif os(Cygwin)
 public typealias sighandler_t = _sig_func_ptr
 
 public var SIG_DFL: sighandler_t? { return nil }
@@ -394,7 +393,7 @@ public var SIG_ERR: sighandler_t {
 public var SIG_HOLD: sighandler_t {
   return unsafeBitCast(2, to: sighandler_t.self)
 }
-#else
+#elseif os(Windows)
 public var SIG_DFL: _crt_signal_t? { return nil }
 public var SIG_IGN: _crt_signal_t {
   return unsafeBitCast(1, to: _crt_signal_t.self)
@@ -402,7 +401,6 @@ public var SIG_IGN: _crt_signal_t {
 public var SIG_ERR: _crt_signal_t {
   return unsafeBitCast(-1, to: _crt_signal_t.self)
 }
-#endif
 #else
 internal var _ignore = _UnsupportedPlatformError()
 #endif
@@ -411,13 +409,13 @@ internal var _ignore = _UnsupportedPlatformError()
 // semaphore.h
 //===----------------------------------------------------------------------===//
 
-#if !os(Windows) || CYGWIN
+#if !os(Windows) 
 /// The value returned by `sem_open()` in the case of failure.
 public var SEM_FAILED: UnsafeMutablePointer<sem_t>? {
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
   // The value is ABI.  Value verified to be correct for OS X, iOS, watchOS, tvOS.
   return UnsafeMutablePointer<sem_t>(bitPattern: -1)
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
   // The value is ABI.  Value verified to be correct on Glibc.
   return UnsafeMutablePointer<sem_t>(bitPattern: 0)
 #else
diff --git a/stdlib/public/Platform/tgmath.swift.gyb b/stdlib/public/Platform/tgmath.swift.gyb
index 6f44204460235..34bd5443fe6fe 100644
--- a/stdlib/public/Platform/tgmath.swift.gyb
+++ b/stdlib/public/Platform/tgmath.swift.gyb
@@ -216,13 +216,13 @@ public func fpclassify(_ value: ${T}) -> Int {
 %if T == 'Double':
 #if os(Linux)
   return Int(__fpclassify(CDouble(value)))
-#elseif os(Windows) && !CYGWIN
+#elseif os(Windows) 
   return Int(_dclass(CDouble(value)))
 #else
   return Int(__fpclassifyd(CDouble(value)))
 #endif
 %else:
-#if os(Windows) && !CYGWIN
+#if os(Windows) 
   return Int(_${f}dclass(${CT}(value)))
 #else
   return Int(__fpclassify${f}(${CT}(value)))
@@ -298,7 +298,7 @@ public func scalbn(_ x: ${T}, _ n: Int) -> ${T} {
 
 % # This is AllFloatTypes not OverlayFloatTypes because of the tuple return.
 % for T, CT, f in AllFloatTypes():
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin)
 @_transparent
 public func lgamma(_ x: ${T}) -> (${T}, Int) {
   var sign = Int32(0)
@@ -306,16 +306,7 @@ public func lgamma(_ x: ${T}) -> (${T}, Int) {
   return (${T}(value), Int(sign))
 }
 #elseif os(Windows)
-#if CYGWIN
-@_transparent
-public func lgamma(_ x: ${T}) -> (${T}, Int) {
-  var sign = Int32(0)
-  let value = lgamma${f}_r(${CT}(x), &sign)
-  return (${T}(value), Int(sign))
-}
-#else
 // TODO(compnerd): implement
-#endif
 #else
 % # On Darwin platform,
 % # The real lgamma_r is not imported because it hides behind macro _REENTRANT.
@@ -360,11 +351,7 @@ public func nan(_ tag: String) -> ${T} {
 @_transparent
 public func jn(_ n: Int, _ x: Double) -> Double {
 #if os(Windows)
-#if CYGWIN
-  return jn(Int32(n), x)
-#else
   return _jn(Int32(n), x)
-#endif
 #else
   return jn(Int32(n), x)
 #endif
@@ -373,11 +360,7 @@ public func jn(_ n: Int, _ x: Double) -> Double {
 @_transparent
 public func yn(_ n: Int, _ x: Double) -> Double {
 #if os(Windows)
-#if CYGWIN
-  return yn(Int32(n), x)
-#else
   return _yn(Int32(n), x)
-#endif
 #else
   return yn(Int32(n), x)
 #endif
diff --git a/stdlib/public/core/CTypes.swift b/stdlib/public/core/CTypes.swift
index 1f59945e662ba..923580a52dc6c 100644
--- a/stdlib/public/core/CTypes.swift
+++ b/stdlib/public/core/CTypes.swift
@@ -43,14 +43,14 @@ public typealias CShort = Int16
 public typealias CInt = Int32
 
 /// The C 'long' type.
-#if os(Windows) && !CYGWIN && arch(x86_64)
+#if os(Windows) && arch(x86_64)
 public typealias CLong = Int32
 #else
 public typealias CLong = Int
 #endif
 
 /// The C 'long long' type.
-#if os(Windows) && !CYGWIN && arch(x86_64)
+#if os(Windows) && arch(x86_64)
 public typealias CLongLong = Int
 #else
 public typealias CLongLong = Int64
diff --git a/stdlib/public/core/FloatingPoint.swift.gyb b/stdlib/public/core/FloatingPoint.swift.gyb
index e34a1c1d56eb1..620c89727c65e 100644
--- a/stdlib/public/core/FloatingPoint.swift.gyb
+++ b/stdlib/public/core/FloatingPoint.swift.gyb
@@ -1465,7 +1465,7 @@ public protocol BinaryFloatingPoint: FloatingPoint, ExpressibleByFloatLiteral {
   /// - Parameter value: A floating-point value.
   init(_ value: Double)
 
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
   /// Creates a new instance from the given value, rounded to the closest
   /// possible representation.
   ///
diff --git a/stdlib/public/core/FloatingPointParsing.swift.gyb b/stdlib/public/core/FloatingPointParsing.swift.gyb
index 1fe02dd4f9317..b64710810b277 100644
--- a/stdlib/public/core/FloatingPointParsing.swift.gyb
+++ b/stdlib/public/core/FloatingPointParsing.swift.gyb
@@ -38,7 +38,7 @@ internal func _isspace_clocale(_ u: UTF16.CodeUnit) -> Bool {
 %   Self = floatName(bits)
 
 % if bits == 80:
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 % end
 
 //===--- Parsing ----------------------------------------------------------===//
diff --git a/stdlib/public/core/FloatingPointTypes.swift.gyb b/stdlib/public/core/FloatingPointTypes.swift.gyb
index d3fa0c6af875c..5b4548b5505b2 100644
--- a/stdlib/public/core/FloatingPointTypes.swift.gyb
+++ b/stdlib/public/core/FloatingPointTypes.swift.gyb
@@ -64,7 +64,7 @@ else:
 }%
 
 % if bits == 80:
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 % end
 
 ${SelfDocComment}
@@ -702,7 +702,7 @@ extension ${Self} : _ExpressibleByBuiltinIntegerLiteral, ExpressibleByIntegerLit
   }
 }
 
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 
 % builtinFloatLiteralBits = 80
 extension ${Self} : _ExpressibleByBuiltinFloatLiteral {
@@ -847,7 +847,7 @@ extension ${Self} {
 %   That = src_type.stdlib_name
 
 %   if srcBits == 80:
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 %   end
 
 %   if srcBits == bits:
diff --git a/stdlib/public/core/Policy.swift b/stdlib/public/core/Policy.swift
index 306d3fcde0f54..8172198e16359 100644
--- a/stdlib/public/core/Policy.swift
+++ b/stdlib/public/core/Policy.swift
@@ -104,7 +104,7 @@ public typealias StringLiteralType = String
 // IEEE Binary64, and we need 1 bit to represent the sign.  Instead of using
 // 1025, we use the next round number -- 2048.
 public typealias _MaxBuiltinIntegerType = Builtin.Int2048
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 public typealias _MaxBuiltinFloatType = Builtin.FPIEEE80
 #else
 public typealias _MaxBuiltinFloatType = Builtin.FPIEEE64
diff --git a/stdlib/public/core/Runtime.swift.gyb b/stdlib/public/core/Runtime.swift.gyb
index 4a097fd74fe2f..c1abf2316606b 100644
--- a/stdlib/public/core/Runtime.swift.gyb
+++ b/stdlib/public/core/Runtime.swift.gyb
@@ -340,7 +340,7 @@ internal struct _Buffer72 {
 % for bits in [ 32, 64, 80 ]:
 
 % if bits == 80:
-#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
+#if !os(Windows) && (arch(i386) || arch(x86_64))
 % end
 
 @_silgen_name("swift_float${bits}ToString")
diff --git a/test/Parse/ConditionalCompilation/x64CygwinTarget.swift b/test/Parse/ConditionalCompilation/x64CygwinTarget.swift
new file mode 100644
index 0000000000000..0af7084f942cd
--- /dev/null
+++ b/test/Parse/ConditionalCompilation/x64CygwinTarget.swift
@@ -0,0 +1,8 @@
+// RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+#if arch(x86_64) && os(Cygwin) && _runtime(_Native)
+class C {}
+var x = C()
+#endif
+var y = x
+
diff --git a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
index 89f0f91b6833d..f89314c1ad6ef 100644
--- a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
@@ -1,5 +1,5 @@
-// RUN: %swift -typecheck %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
-// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+// RUN: %swift -typecheck %s -verify -D FOO -D BAR -target x86_64-unknown-windows-msvc -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-msvc
 
 #if arch(x86_64) && os(Windows) && _runtime(_Native) && _endian(little)
 class C {}