[Clang] Error on extraneous template headers by default.

clang/docs/ReleaseNotes.rst

@@ -49,6 +49,15 @@ C++ Specific Potentially Breaking Changes
   few users and can be written as ``__is_same(__remove_cv(T), decltype(nullptr))``,
   which GCC supports as well.
 
+- 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

@@ -5426,7 +5426,8 @@ 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

@@ -128,11 +128,13 @@ 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>
 {};
 
+template<> void f(auto); // expected-error{{extraneous template parameter list}}
+
 namespace N1 {
 
   template<typename T> struct X7 { }; // expected-note{{here}}