compilers: Fix has_function check for builtins

Use a single check for both cases when we have includes and when we
don't. This way we ensure three things:

1. Built-in checks are 100% reliable with clang and on macOS since clang
   implements __has_builtin
2. When the #include is present, this ensures that __builtin_func is not
   checked for (because of MSYS, and because it is faster)
3. We fallback to checking __builtin_func when all else fails

mesonbuild/compilers.py

@@ -1045,33 +1045,34 @@ class CCompiler(Compiler):
 
         if self.links(templ.format(**fargs), env, extra_args, dependencies):
             return True
+
+        # Detect function as a built-in
+        #
         # Some functions like alloca() are defined as compiler built-ins which
-        # are inlined by the compiler, so look for __builtin_symbol in the libc
-        # if there's no #include-s in prefix which would've #define-d the
-        # symbol correctly. If there is a #include, just check for the symbol
-        # directly. This is needed because the above #undef fancy footwork
-        # doesn't work for builtins.
-        # This fixes instances such as #1083 where MSYS2 defines
-        # __builtin_posix_memalign in the C library but doesn't define
-        # posix_memalign in the headers to point to that builtin which results
-        # in an invalid detection.
-        if '#include' not in prefix:
-            # Detect function as a built-in
-            fargs['func'] = '__builtin_' + fargs['func']
-            code = '''
-            int main() {{
-            #ifdef __has_builtin
-                #if !__has_builtin({func})
-                    #error "built-in {func} not found"
-                #endif
+        # are inlined by the compiler and you can't take their address, so we
+        # need to look for them differently. On nice compilers like clang, we
+        # can just directly use the __has_builtin() macro.
+        fargs['no_includes'] = '#include' not in prefix
+        t = '''{prefix}
+        int main() {{
+        #ifdef __has_builtin
+            #if !__has_builtin(__builtin_{func})
+                #error "__builtin_{func} not found"
+            #endif
+        #elif ! defined({func})
+            /* Check for __builtin_{func} only if no includes were added to the
+             * prefix above, which means no definition of {func} can be found.
+             * We would always check for this, but we get false positives on
+             * MSYS2 if we do. Their toolchain is broken, but we can at least
+             * give them a workaround. */
+            #if {no_includes:d}
+                __builtin_{func};
             #else
-                {func};
+                #error "No definition for __builtin_{func} found in the prefix"
             #endif
-            }}'''
-        else:
-            # Directly look for the function itself
-            code = '{prefix}\n' + stubs_fail + '\nint main() {{ {func}; }}'
-        return self.links(code.format(**fargs), env, extra_args, dependencies)
+        #endif
+        }}'''
+        return self.links(t.format(**fargs), env, extra_args, dependencies)
 
     def has_members(self, typename, membernames, prefix, env, extra_args=None, dependencies=None):
         if extra_args is None:

test cases/common/43 has function/meson.build

@@ -4,6 +4,10 @@ host_system = host_machine.system()
 
 # This is used in the `test_compiler_check_flags_order` unit test
 unit_test_args = '-I/tmp'
+defines_has_builtin = '''#ifndef __has_builtin
+#error "no __has_builtin"
+#endif
+'''
 compilers = [meson.get_compiler('c'), meson.get_compiler('cpp')]
 
 foreach cc : compilers
@@ -40,20 +44,33 @@ foreach cc : compilers
   # We can't check for the C library used here of course, but if it's not
   # implemented in glibc it's probably not implemented in any other 'slimmer'
   # C library variants either, so the check should be safe either way hopefully.
-  if host_system == 'linux'
+  if host_system == 'linux' or host_system == 'darwin'
     assert (cc.has_function('poll', prefix : '#include <poll.h>',
                             args : unit_test_args),
             'couldn\'t detect "poll" when defined by a header')
     lchmod_prefix = '#include <sys/stat.h>\n#include <unistd.h>'
-    assert (not cc.has_function('lchmod', prefix : lchmod_prefix,
-                                args : unit_test_args),
-            '"lchmod" check should have failed')
+    if host_system == 'linux'
+      assert (not cc.has_function('lchmod', prefix : lchmod_prefix,
+                                  args : unit_test_args),
+              '"lchmod" check should have failed')
+    else
+      # macOS and *BSD have lchmod
+      assert (cc.has_function('lchmod', prefix : lchmod_prefix,
+                                  args : unit_test_args),
+              '"lchmod" check should have succeeded')
+    endif
     # Check that built-ins are found properly both with and without headers
     assert(cc.has_function('alloca', args : unit_test_args),
-           'built-in alloca must be found on Linux')
+           'built-in alloca must be found on ' + host_system)
     assert(cc.has_function('alloca', prefix : '#include <alloca.h>',
            args : unit_test_args),
-           'built-in alloca must be found on Linux with #include')
+           'built-in alloca must be found with #include')
+    if not cc.compiles(defines_has_builtin, args : unit_test_args)
+      assert(not cc.has_function('alloca',
+             prefix : '#include <alloca.h>\n#undef alloca',
+             args : unit_test_args),
+             'built-in alloca must not be found with #include and #undef')
+    endif
   endif
 
   # For some functions one needs to define _GNU_SOURCE before including the