Fix visibility attribute support check for GCC on Windows

has_function_attribute() depends on -Wattributes being emitted when an attribute
is not supported by the compiler. In case of GCC on Window (at least) there is no
warning in case the attribute is used on a declaration. Only once there is also a
function definition does it emit a warning like:

a.c: In function ‘foo’:
a.c:8:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
    8 | }

To fix this add a dummy function definition to all visibility compiler checks in meson.

The tests in "197 function attributes" only checked for positive return result on on-msvc
compilers, except one special case for dllexport/dllimport. Refactor the tests a bit so
one can specify also a negative expected result, and add tests for all visibility attribute
variants.

2 files changed, 36 insertions, 21 deletions

diff --git a/mesonbuild/compilers/c_function_attributes.py b/mesonbuild/compilers/c_function_attributes.py
index 71ee9b22a..eec872b5f 100644
--- a/mesonbuild/compilers/c_function_attributes.py
+++ b/mesonbuild/compilers/c_function_attributes.py
@@ -103,17 +103,17 @@ C_FUNC_ATTRIBUTES = {
     'vector_size':
         '__attribute__((vector_size(32))); int foo(void) { return 0; }',
     'visibility': '''
-        int foo_def(void) __attribute__((visibility("default")));
-        int foo_hid(void) __attribute__((visibility("hidden")));
-        int foo_int(void) __attribute__((visibility("internal")));''',
+        int foo_def(void) __attribute__((visibility("default"))); int foo_def(void) { return 0; }
+        int foo_hid(void) __attribute__((visibility("hidden"))); int foo_hid(void) { return 0; }
+        int foo_int(void) __attribute__((visibility("internal"))); int foo_int(void) { return 0; }''',
     'visibility:default':
-        'int foo(void) __attribute__((visibility("default")));',
+        'int foo(void) __attribute__((visibility("default"))); int foo(void) { return 0; }',
     'visibility:hidden':
-        'int foo(void) __attribute__((visibility("hidden")));',
+        'int foo(void) __attribute__((visibility("hidden"))); int foo(void) { return 0; }',
     'visibility:internal':
-        'int foo(void) __attribute__((visibility("internal")));',
+        'int foo(void) __attribute__((visibility("internal"))); int foo(void) { return 0; }',
     'visibility:protected':
-        'int foo(void) __attribute__((visibility("protected")));',
+        'int foo(void) __attribute__((visibility("protected"))); int foo(void) { return 0; }',
     'warning':
         'int foo(void) __attribute__((warning("")));',
     'warn_unused_result':
diff --git a/test cases/common/197 function attributes/meson.build b/test cases/common/197 function attributes/meson.build
index 0d5c04ed1..8ef6b741d 100644
--- a/test cases/common/197 function attributes/meson.build
+++ b/test cases/common/197 function attributes/meson.build
@@ -23,7 +23,8 @@ if c.get_id() == 'pgi'
   error('MESON_SKIP_TEST: PGI supports its own set of features, will need a separate list for PGI to test it.')
 endif
 
-expected_result = not ['msvc', 'clang-cl', 'intel-cl'].contains(c.get_id())
+expected = {}
+expected_default = not ['msvc', 'clang-cl', 'intel-cl'].contains(c.get_id())
 
 # Q: Why is ifunc not in this list or any of the below lists?
 # A: It's too damn hard to figure out if you actually support it, since it
@@ -56,8 +57,24 @@ attributes = [
   'vector_size',
   'warn_unused_result',
   'weak',
+  'dllexport',
+  'dllimport',
 ]
 
+expected += {
+  'dllexport': ['windows', 'cygwin'].contains(host_machine.system()),
+  'dllimport': ['windows', 'cygwin'].contains(host_machine.system()),
+}
+
+if c.get_id() == 'gcc' and ['windows', 'cygwin'].contains(host_machine.system())
+  expected += {
+    'visibility': false,
+    'visibility:hidden': false,
+    'visibility:internal': false,
+    'visibility:protected': false,
+  }
+endif
+
 if c.get_id() != 'intel'
   # not supported by icc as of 19.0.0
   attributes += 'weakref'
@@ -67,6 +84,10 @@ endif
 if host_machine.system() != 'darwin'
   attributes += 'alias'
   attributes += 'visibility'
+  attributes += 'visibility:default'
+  attributes += 'visibility:hidden'
+  attributes += 'visibility:internal'
+  attributes += 'visibility:protected'
   attributes += 'alloc_size'
 endif
 
@@ -94,24 +115,18 @@ endif
 if get_option('mode') == 'single'
   foreach a : attributes
     x = c.has_function_attribute(a)
+    expected_result = expected.get(a, expected_default)
     assert(x == expected_result, '@0@: @1@'.format(c.get_id(), a))
     x = cpp.has_function_attribute(a)
     assert(x == expected_result, '@0@: @1@'.format(cpp.get_id(), a))
   endforeach
-
-  win_expect = ['windows', 'cygwin'].contains(host_machine.system())
-  foreach a : ['dllexport', 'dllimport']
-    assert(c.has_function_attribute(a) == win_expect,
-          '@0@: @1@'.format(c.get_id(), a))
-    assert(cpp.has_function_attribute(a) == win_expect,
-          '@0@: @1@'.format(cpp.get_id(), a))
-  endforeach
 else
-  if not ['msvc', 'clang-cl', 'intel-cl'].contains(c.get_id())
-    multi_expected = attributes
-  else
-    multi_expected = []
-  endif
+  multi_expected = []
+  foreach a : attributes
+    if expected.get(a, expected_default)
+      multi_expected += a
+    endif
+  endforeach
 
   multi_check = c.get_supported_function_attributes(attributes)
   assert(multi_check == multi_expected, 'get_supported_function_arguments works (C)')