Workaround for now. I should have specified in the bug that I overlooked it because my version of clang seems to use C99 for Objective-C by default, but not for Objective-C++. I'll do that when I'm next online. Anyway, now to write uiControl functionality tests.

test/controls_darwin_errors.m

@@ -0,0 +1,22 @@
+// 16 june 2019
+#import "test_darwin.h"
+
+// TODO move this back to C++ once https://github.com/mesonbuild/meson/issues/5495 is resolved
+
+static void doRCTWrongSizeTest(void)
+{
+	uiControlOSVtable vtable;
+
+	memset(&vtable, 0, sizeof (uiControlOSVtable));
+	vtable.Size = 1;
+	uiRegisterControlType("name", testVtable(), &vtable, 0);
+}
+
+const struct checkErrorCase controlOSVtableCases[] = {
+	{
+		"uiRegisterControlType() with OS vtable with wrong size",
+		doRCTWrongSizeTest,
+		"uiRegisterControlType(): wrong size 1 for uiControlOSVtable",
+	},
+	{ NULL, NULL, NULL },
+};

test/controls_darwin_errors.mm

@@ -1,17 +0,0 @@
-// 16 june 2019
-#import "test_darwin.h"
-
-const struct checkErrorCase controlOSVtableCases[] = {
-	{
-		"uiRegisterControlType() with OS vtable with wrong size",
-		[](void) {
-			uiControlOSVtable vtable;
-	
-			memset(&vtable, 0, sizeof (uiControlOSVtable));
-			vtable.Size = 1;
-			uiRegisterControlType("name", testVtable(), &vtable, 0);
-		},
-		"uiRegisterControlType(): wrong size 1 for uiControlOSVtable",
-	},
-	{ NULL, NULL, NULL },
-};

test/meson.build

@@ -16,11 +16,9 @@ if libui_OS == 'windows'
 		'controls_windows.c',
 	]
 elif libui_OS == 'darwin'
-	add_languages('objcpp',
-		required: true)
 	libui_test_sources += [
 		'controls_darwin.m',
-		'controls_darwin_errors.mm',
+		'controls_darwin_errors.m',
 	]
 else
 	libui_test_sources += [