Update test_implicit_func for LLVM 16 (#17623)

In LLVM 16, -Wincompatible-function-pointer types has been upgraded to an error
by default. The functional change in this PR is to update test_implicit_func
to incorporate this. (Currently it accepts either an error or a warning to avoid a complex DEPS roll).
I also minimized the test to only test the behavior that is unique to emscripten (namely, the fact that
-Wimplicit-function-declaration is an error in gnu89 mode rather than a warning as it is upstream).

Author: dschuff

emcc.py

@@ -870,7 +870,7 @@ def get_clang_flags(user_args):
 cflags = None
 
 
-def get_cflags(user_args):
+def get_cflags(user_args, is_cxx):
   global cflags
   if cflags:
     return cflags
@@ -906,7 +906,9 @@ def get_cflags(user_args):
   #   -Wno-error=implicit-function-declaration
   # or disable even a warning about it with
   #   -Wno-implicit-function-declaration
-  cflags += ['-Werror=implicit-function-declaration']
+  # This is already an error in C++ so we don't need to inject extra flags.
+  if not is_cxx:
+    cflags += ['-Werror=implicit-function-declaration']
 
   ports.add_cflags(cflags, settings)
 
@@ -1133,7 +1135,7 @@ def run(args):
 
   passthrough_flags = ['-print-search-dirs', '-print-libgcc-file-name']
   if any(a in args for a in passthrough_flags) or any(a.startswith('-print-file-name=') for a in args):
-    return run_process([clang] + args + get_cflags(args), check=False).returncode
+    return run_process([clang] + args + get_cflags(args, run_via_emxx), check=False).returncode
 
   ## Process argument and setup the compiler
   state = EmccState(args)
@@ -2728,7 +2730,7 @@ def get_compiler(src_file):
     return CC
 
   def get_clang_command(src_file):
-    return get_compiler(src_file) + get_cflags(state.orig_args) + compile_args + [src_file]
+    return get_compiler(src_file) + get_cflags(state.orig_args, use_cxx(src_file)) + compile_args + [src_file]
 
   def get_clang_command_preprocessed(src_file):
     return get_compiler(src_file) + get_clang_flags(state.orig_args) + compile_args + [src_file]

test/other/test_implicit_func.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+int main() {
+    printf("hello %d\n", strnlen("waka", 2)); // Implicit declaration, no header, for strnlen
+    int (*my_strnlen)(char*, ...) = strnlen;
+    printf("hello %d\n", my_strnlen("shaka", 2));
+    return 0;
+}

test/test_other.py

@@ -3978,45 +3978,17 @@ def test_global_inits(self):
       self.assertContainedIf('globalCtors', src, has_global)
 
   def test_implicit_func(self):
-    create_file('src.c', r'''
-#include <stdio.h>
-int main()
-{
-    printf("hello %d\n", strnlen("waka", 2)); // Implicit declaration, no header, for strnlen
-    int (*my_strnlen)(char*, ...) = strnlen;
-    printf("hello %d\n", my_strnlen("shaka", 2));
-    return 0;
-}
-''')
-
-    IMPLICIT_WARNING = "warning: implicit declaration of function 'strnlen' is invalid in C99"
-    IMPLICIT_WARNING_NEW = "warning: call to undeclared function 'strnlen'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]"
-
-    IMPLICIT_ERROR = "error: implicit declaration of function 'strnlen' is invalid in C99"
-    IMPLICIT_ERROR_NEW = "error: call to undeclared function 'strnlen'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]"
-
-    INCOMPATIBLE_WARNINGS = ('warning: incompatible pointer types', 'warning: incompatible function pointer types')
-    IMPLICIT_WARNINGS = (IMPLICIT_WARNING, IMPLICIT_WARNING_NEW)
-    IMPLICIT_ERRORS = (IMPLICIT_ERROR, IMPLICIT_ERROR_NEW)
-
-    for opts, expected, compile_expected in [
-      ([], None, [IMPLICIT_ERRORS]),
-      # turn error into warning
-      (['-Wno-error=implicit-function-declaration'], ['hello '], [IMPLICIT_WARNINGS]),
-      # turn error into nothing at all (runtime output is incorrect)
-      (['-Wno-implicit-function-declaration'], ['hello '], []),
-    ]:
-      print(opts, expected)
-      try_delete('a.out.js')
-      stderr = self.run_process([EMCC, 'src.c'] + opts, stderr=PIPE, check=False).stderr
-      for ce in compile_expected + [INCOMPATIBLE_WARNINGS]:
-        self.assertContained(ce, stderr)
-      if expected is None:
-        self.assertNotExists('a.out.js')
-      else:
-        output = self.run_js('a.out.js')
-        for e in expected:
-          self.assertContained(e, output)
+    # EMCC makes -Wimplict-function-declaration an error by default in all modes. Upstream LLVM
+    # emits a warning in gnu89 mode, but otherwise emcc's behavior is identical to upstream.
+    IMPLICIT_C89 = "error: implicit declaration of function 'strnlen'"
+    # Also check for -Wincompatible-function-pointer-types (it became an error in LLVM 16)
+    INCOMPATIBLE = ': incompatible function pointer types'
+
+    try_delete('implicit_func.o')
+    stderr = self.expect_fail(
+        [EMCC, path_from_root('test/other/test_implicit_func.c'), '-c', '-o', 'implicit_func.o', '-std=gnu89'])
+    self.assertContained(IMPLICIT_C89, stderr)
+    self.assertContained(INCOMPATIBLE, stderr)
 
   @requires_native_clang
   def test_bad_triple(self):