Increase severity level of -Wunguarded-availability-new on XCode 9

mesonbuild/compilers/mixins/clang.py

@@ -101,13 +101,24 @@ def has_function(self, funcname: str, prefix: str, env: 'Environment', *,
                      dependencies: T.Optional[T.List['Dependency']] = None) -> T.Tuple[bool, bool]:
         if extra_args is None:
             extra_args = []
-        # Starting with XCode 8, we need to pass this to force linker
-        # visibility to obey OS X/iOS/tvOS minimum version targets with
-        # -mmacosx-version-min, -miphoneos-version-min, -mtvos-version-min etc.
-        # https://github.com/Homebrew/homebrew-core/issues/3727
         # TODO: this really should be communicated by the linker
-        if isinstance(self.linker, AppleDynamicLinker) and mesonlib.version_compare(self.version, '>=8.0'):
-            extra_args.append('-Wl,-no_weak_imports')
+        if isinstance(self.linker, AppleDynamicLinker):
+            # Starting with XCode 8, we need to pass this to force linker
+            # visibility to obey OS X/iOS/tvOS minimum version targets with
+            # -mmacosx-version-min, -miphoneos-version-min, -mtvos-version-min etc.
+            # https://github.com/Homebrew/homebrew-core/issues/3727
+            if mesonlib.version_compare(self.version, '>=8.0'):
+                extra_args.append('-Wl,-no_weak_imports')
+            # XCode 9 introduced two new warnings:
+            # -Wunguarded-availability and Wunguarded-availability-new
+            # The former is off by default, the latter on by default,
+            # -Wunguarded-availability-new only warns about APIs introduced
+            # in or after macOS 10.13, iOS 11, tvOS 11 or watchOS 4.
+            # Since macOS 10.13 was EOLed on 1 Dec 2020, there's no need to
+            # increase the severity level of the former warning.
+            # https://github.com/lovell/sharp/issues/3438
+            if mesonlib.version_compare(self.version, '>=9.0'):
+                extra_args.append('-Werror=unguarded-availability-new')
         return super().has_function(funcname, prefix, env, extra_args=extra_args,
                                     dependencies=dependencies)