[CMake] [Shims] [Darwin] Support building with and for the new Apple SDKs

The Apple SDKs have been providing the Darwin overlay since macOS 10.14.4, iOS 12.2, et al. More recently the SDK version has diverged from the Swift version making them incompatible. Stop building the overlay from Swift.

macOS 15.0, iOS 18.0, et al has started unconditionally declaring `lgamma_r` in <math.h>, add a __has_include in SwiftShims to detect that and not redeclare it when building against those SDK versions.

Author: ian-twilightcoder

CMakeLists.txt

@@ -244,9 +244,16 @@ option(SWIFT_BUILD_CLANG_OVERLAYS
   "Build Swift overlays for the clang builtin modules"
   TRUE)
 
-option(SWIFT_BUILD_DYNAMIC_SDK_OVERLAY
-    "Build dynamic variants of the Swift SDK overlay"
-    TRUE)
+# The SDK overlay is provided by the SDK itself on Darwin platforms.
+if (SWIFT_HOST_VARIANT_SDK IN_LIST SWIFT_DARWIN_PLATFORMS)
+  option(SWIFT_BUILD_DYNAMIC_SDK_OVERLAY
+      "Build dynamic variants of the Swift SDK overlay"
+      FALSE)
+else()
+  option(SWIFT_BUILD_DYNAMIC_SDK_OVERLAY
+      "Build dynamic variants of the Swift SDK overlay"
+      TRUE)
+endif()
 
 option(SWIFT_BUILD_STATIC_SDK_OVERLAY
     "Build static variants of the Swift SDK overlay"

stdlib/public/SwiftShims/swift/shims/LibcShims.h

@@ -170,11 +170,16 @@ long double _stdlib_squareRootl(long double _self) {
 // Apple's math.h does not declare lgamma_r() etc by default, but they're
 // unconditionally exported by libsystem_m.dylib in all OS versions that
 // support Swift development; we simply need to provide declarations here.
-#if defined(__APPLE__)
+// In the macOS 15.0, iOS 18.0, et al SDKs, math.h unconditionally declares
+// lgamma_r() when building for Swift. Detect those SDKs by checking for a
+// header which was added in those versions. (Redeclaring the function
+// would cause an error where `lgamma_r` is ambiguous between the SDK
+// `_math.lgamma_r` and this `SwiftShims.lgamma_r`.)
+#if defined(__APPLE__) && !__has_include(<_modules/_math_h.h>)
 float lgammaf_r(float x, int *psigngam);
 double lgamma_r(double x, int *psigngam);
 long double lgammal_r(long double x, int *psigngam);
-#endif // defined(__APPLE__)
+#endif // defined(__APPLE__) && !__has_include(<_modules/_math_h.h>)
 
 #ifdef __cplusplus
 } // extern "C"

utils/build_swift/build_swift/driver_arguments.py

@@ -8,6 +8,7 @@
 
 import multiprocessing
 import os
+import platform
 
 import android.adb.commands
 
@@ -1136,7 +1137,7 @@ def create_argument_parser():
            help='build static variants of the Swift standard library')
 
     option('--build-swift-dynamic-sdk-overlay', toggle_true,
-           default=True,
+           default=platform.system() != "Darwin",
            help='build dynamic variants of the Swift SDK overlay')
 
     option('--build-swift-static-sdk-overlay', toggle_true,

utils/build_swift/tests/expected_options.py

@@ -8,6 +8,7 @@
 
 
 import multiprocessing
+import platform
 
 from build_swift import argparse
 from build_swift import defaults
@@ -79,7 +80,7 @@
     'build_runtime_with_host_compiler': False,
     'build_stdlib_deployment_targets': ['all'],
     'build_subdir': None,
-    'build_swift_dynamic_sdk_overlay': True,
+    'build_swift_dynamic_sdk_overlay': platform.system() != "Darwin",
     'build_swift_dynamic_stdlib': True,
     'build_swift_inspect': False,
     'build_swift_private_stdlib': True,