[Clang] Error on extraneous template headers by default. (#104046)

As discussed here

#99296 (comment)

Fixes #99296
Fixes #50294

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -65,6 +65,15 @@ C++ Specific Potentially Breaking Changes
   `-Wno-enum-constexpr-conversion`, to allow for a transition period for users.
   Now, in Clang 20, **it is no longer possible to suppress the diagnostic**.
 
+- Extraneous template headers are now ill-formed by default.
+  This error can be disable with ``-Wno-error=extraneous-template-head``.
+
+  .. code-block:: c++
+
+    template <> // error: extraneous template head
+    template <typename T>
+    void f();
+    
 ABI Changes in This Version
 ---------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5428,7 +5428,8 @@ def err_template_spec_extra_headers : Error<
   "extraneous template parameter list in template specialization or "
   "out-of-line template definition">;
 def ext_template_spec_extra_headers : ExtWarn<
-  "extraneous template parameter list in template specialization">;
+  "extraneous template parameter list in template specialization">,
+   InGroup<DiagGroup<"extraneous-template-head">>, DefaultError;
 def note_explicit_template_spec_does_not_need_header : Note<
   "'template<>' header not required for explicitly-specialized class %0 "
   "declared here">;

clang/test/Misc/warning-flags.c

@@ -18,14 +18,13 @@ This test serves two purposes:
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (65):
+CHECK: Warnings without flags (64):
 
 CHECK-NEXT:   ext_expected_semi_decl_list
 CHECK-NEXT:   ext_missing_whitespace_after_macro_name
 CHECK-NEXT:   ext_new_paren_array_nonconst
 CHECK-NEXT:   ext_plain_complex
 CHECK-NEXT:   ext_template_arg_extra_parens
-CHECK-NEXT:   ext_template_spec_extra_headers
 CHECK-NEXT:   ext_typecheck_cond_incompatible_operands
 CHECK-NEXT:   ext_typecheck_ordered_comparison_of_pointer_integer
 CHECK-NEXT:   ext_using_undefined_std

clang/test/SemaTemplate/temp_explicit.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -pedantic -Wc++11-compat %s
 // RUN: %clang_cc1 -fsyntax-only -verify -pedantic -Wc++11-compat -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -pedantic -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic -std=c++20 %s
 //
 // Tests explicit instantiation of templates.
 template<typename T, typename U = T> class X0 { };
@@ -128,11 +129,15 @@ struct Foo<int> // expected-note{{header not required for explicitly-specialized
     {};
 };
 
-template <> // expected-warning{{extraneous template parameter list}}
+template <> // expected-error{{extraneous template parameter list}}
 template <>
 struct Foo<int>::Bar<void>
 {};
 
+#if __cplusplus >= 202002L
+template<> void f(auto); // expected-error{{extraneous template parameter list}}
+#endif
+
 namespace N1 {
 
   template<typename T> struct X7 { }; // expected-note{{here}}