compilers: Check if GCC has support for ObjC and/or ObjC++

Since this is optional, we should not accept that GCC is a valid ObjC or
G++ is a valid ObjC++ Compiler unless we've tested that they can
actually do a basic compile.

This requires fixing a number of tests that have broken assumptions. In
some cases I've split tests where issues with one language would hide
the other. It would be great if we had a competent test framework that
allowed subtests to skip, unfortunately we have python's unittest
instead. Because of that we can't avoid extra tests by use of subtests.

mesonbuild/compilers/detect.py

@@ -877,9 +877,13 @@ def _detect_objc_or_objcpp_compiler(env: 'Environment', lang: str, for_machine:
             version = _get_gnu_version_from_defines(defines)
             comp = objc.GnuObjCCompiler if lang == 'objc' else objcpp.GnuObjCPPCompiler
             linker = guess_nix_linker(env, compiler, comp, version, for_machine)
-            return comp(
+            c = comp(
                 ccache, compiler, version, for_machine, is_cross, info,
                 defines, linker=linker)
+            if not c.compiles('int main(void) { return 0; }', env)[0]:
+                popen_exceptions[join_args(compiler)] = f'GCC was not built with support for {"objective-c" if lang == "objc" else "objective-c++"}'
+                continue
+            return c
         if 'clang' in out:
             linker = None
             defines = _get_clang_compiler_defines(compiler, lang)

unittests/failuretests.py

@@ -238,19 +238,26 @@ def test_dependency_invalid_method(self):
         '''
         self.assertMesonRaises(code, ".* is not a config-tool dependency")
 
-    def test_objc_cpp_detection(self):
+    def test_objc_detection(self) -> None:
         '''
         Test that when we can't detect objc or objcpp, we fail gracefully.
         '''
         env = get_fake_env()
         try:
             detect_objc_compiler(env, MachineChoice.HOST)
+        except EnvironmentException:
+            self.assertMesonRaises("add_languages('objc')", r"(Unknown compiler|GCC was not built with support)")
+        else:
+            raise unittest.SkipTest('Working objective-c Compiler found, cannot test error.')
+
+    def test_objcpp_detection(self) -> None:
+        env = get_fake_env()
+        try:
             detect_objcpp_compiler(env, MachineChoice.HOST)
         except EnvironmentException:
-            code = "add_languages('objc')\nadd_languages('objcpp')"
-            self.assertMesonRaises(code, "Unknown compiler")
-            return
-        raise unittest.SkipTest("objc and objcpp found, can't test detection failure")
+            self.assertMesonRaises("add_languages('objcpp')", r"(Unknown compiler|GCC was not built with support)")
+        else:
+            raise unittest.SkipTest('Working objective-c++ Compiler found, cannot test error.')
 
     def test_subproject_variables(self):
         '''

unittests/machinefiletests.py

@@ -23,7 +23,7 @@
 import mesonbuild.environment
 import mesonbuild.coredata
 import mesonbuild.modules.gnome
-
+from mesonbuild import mesonlib
 from mesonbuild import machinefile
 
 from mesonbuild.mesonlib import (
@@ -275,7 +275,12 @@ def cb(comp):
             if not is_real_gnu_compiler(shutil.which('gcc')):
                 raise SkipTest('Only one compiler found, cannot test.')
             return 'gcc', 'gcc'
-        self.helper_for_compiler('objc', cb)
+        try:
+            self.helper_for_compiler('objc', cb)
+        except mesonlib.EnvironmentException as e:
+            if 'GCC was not built with support for objective-c' in str(e):
+                raise unittest.SkipTest("GCC doesn't support objective-c, test cannot run")
+            raise
 
     @skip_if_not_language('objcpp')
     @skip_if_env_set('OBJCXX')
@@ -288,7 +293,12 @@ def cb(comp):
             if not is_real_gnu_compiler(shutil.which('g++')):
                 raise SkipTest('Only one compiler found, cannot test.')
             return 'g++', 'gcc'
-        self.helper_for_compiler('objcpp', cb)
+        try:
+            self.helper_for_compiler('objcpp', cb)
+        except mesonlib.EnvironmentException as e:
+            if 'GCC was not built with support for objective-c++' in str(e):
+                raise unittest.SkipTest("G++ doesn't support objective-c++, test cannot run")
+            raise
 
     @skip_if_not_language('d')
     @skip_if_env_set('DC')