fix bug that undefined internal is a warning only for -pedantic-errors

clang/docs/ReleaseNotes.rst

@@ -554,6 +554,9 @@ Attribute Changes in Clang
 
 Improvements to Clang's diagnostics
 -----------------------------------
+- Clang now emits an error instead of a warning for `undefined_internal`
+  when compiling with `-pedantic-errors` to conform to the C standard
+
 - Clang now applies syntax highlighting to the code snippets it
   prints.
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6013,7 +6013,7 @@ def note_deleted_assign_field : Note<
   "because field %2 is of %select{reference|const-qualified}4 type %3">;
 
 // These should be errors.
-def warn_undefined_internal : Warning<
+def warn_undefined_internal : ExtWarn<
   "%select{function|variable}0 %q1 has internal linkage but is not defined">,
   InGroup<DiagGroup<"undefined-internal">>;
 def err_undefined_internal_type : Error<

clang/test/Sema/undefined-internal-alignof.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -pedantic-errors
+
+static void f(void); // expected-no-error
+
+int main(void)
+{
+    int i = _Alignof(f);
+    // expected-error@-1 {{'_Alignof' applied to an expression is a GNU extension}}
+    // expected-error@-2 {{invalid application of 'alignof' to a function type}}
+}

clang/test/Sema/undefined-internal-basic.c

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -pedantic-errors
+
+static void f(void); // expected-error {{function 'f' has internal linkage but is not defined}}
+
+int main(void)
+{
+    f;
+    // expected-note@-1 {{used here}}
+    // expected-warning@-2 {{expression result unused}}
+}
+

clang/test/Sema/undefined-internal-generic-selection.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -pedantic-errors
+
+static void *f(void); // expected-error {{function 'f' has internal linkage but is not defined}}
+
+int main(void)
+{
+    int j = _Generic(&f, void *(*)(void): 10, default: 20);
+    // expected-no-diagnostic@-1
+
+    void *k = _Generic(&f, void *(*)(void): f(), default: 20);
+    // expected-note@-1 {{used here}}
+}

clang/test/Sema/undefined-internal-sizeof.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -pedantic-errors
+
+// expected-no-diagnostics
+
+static int f(void);
+
+int main(void)
+{
+    int x = sizeof f();
+}

clang/test/Sema/undefined-internal-typeof-c23.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c23 -pedantic-errors
+
+// expected-no-diagnostics
+
+static int f(void);
+
+int main(void)
+{
+    typeof(f()) x;
+}