mpich-clang13 @4.2.2: error: _Float16 is not supported on this target

[ryandesign (Ryan Carsten Schmidt)]

​https://build.macports.org/builders/ports-15_x86_64-builder/builds/3399/steps/install-port/logs/stdio

In file included from json_tokener.c:19:
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:614:27: error: _Float16 is not supported on this target
extern _Float16 __fabsf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:614:8: error: _Float16 is not supported on this target
extern _Float16 __fabsf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:615:28: error: _Float16 is not supported on this target
extern _Float16 __hypotf16(_Float16, _Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:615:38: error: _Float16 is not supported on this target
extern _Float16 __hypotf16(_Float16, _Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                                     ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:615:8: error: _Float16 is not supported on this target
extern _Float16 __hypotf16(_Float16, _Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:616:27: error: _Float16 is not supported on this target
extern _Float16 __sqrtf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:616:8: error: _Float16 is not supported on this target
extern _Float16 __sqrtf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:617:27: error: _Float16 is not supported on this target
extern _Float16 __ceilf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:617:8: error: _Float16 is not supported on this target
extern _Float16 __ceilf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:618:28: error: _Float16 is not supported on this target
extern _Float16 __floorf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:618:8: error: _Float16 is not supported on this target
extern _Float16 __floorf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:619:27: error: _Float16 is not supported on this target
extern _Float16 __rintf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:619:8: error: _Float16 is not supported on this target
extern _Float16 __rintf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:620:28: error: _Float16 is not supported on this target
extern _Float16 __roundf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:620:8: error: _Float16 is not supported on this target
extern _Float16 __roundf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:621:28: error: _Float16 is not supported on this target
extern _Float16 __truncf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:621:8: error: _Float16 is not supported on this target
extern _Float16 __truncf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:622:31: error: _Float16 is not supported on this target
extern _Float16 __copysignf16(_Float16, _Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                              ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/math.h:622:41: error: _Float16 is not supported on this target
extern _Float16 __copysignf16(_Float16, _Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
                                        ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]

This is weird to me because the __API_AVAILABLE statement says it's available on macOS 15.0 and we're running macOS 15.0 and as far as I can tell the deployment target is set to 15.0.