[clang] [clang] Diagnose config_macros before building modules (PR #83641)

[Volodymyr Sapsai via cfe-commits]

================
@@ -1,3 +1,39 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -x objective-c -fmodules-cache-path=%t -DWANT_FOO=1 -emit-module -fmodule-name=config %t/module.modulemap
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %t -DWANT_FOO=1 %t/config.m -verify
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %t -DTEST_ERROR %t/config_error.m -verify
+
+//--- module.modulemap
+
+module config {
+  header "config.h"
+  config_macros [exhaustive] WANT_FOO, WANT_BAR
+}
+
+module config_error {
+  header "config_error.h"
+  config_macros SOME_VALUE
+}
+
+//--- config.h
+
+#ifdef WANT_FOO
+int* foo(void);
+#endif
+
+#ifdef WANT_BAR
+char *bar(void);
+#endif
+
+//--- config_error.h
+
+struct my_thing {
+  char buf[SOME_VALUE];
----------------
vsapsai wrote:

Don't think it is worth verifying in the test but we have a diagnostic
```
error: use of undeclared identifier 'SOME_VALUE'
```
which is still useful when we have no corresponding macro defined anywhere.

https://github.com/llvm/llvm-project/pull/83641