PR46231: Promote diagnostic for 'template<...>;' from ExtWarn to Error.

No other compiler accepts this as an extension, not even in permissive
mode. We're not doing anyone any favors by allowing this, and it's
unlikely to be at all common, even in Clang-only code, in the wild.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -793,6 +793,7 @@ def ext_main_used : Extension<
 /// parser diagnostics
 def ext_no_declarators : ExtWarn<"declaration does not declare anything">,
   InGroup<MissingDeclarations>;
+def err_no_declarators : Error<"declaration does not declare anything">;
 def ext_typedef_without_a_name : ExtWarn<"typedef requires a name">,
   InGroup<MissingDeclarations>;
 def err_typedef_not_identifier : Error<"typedef name must be an identifier">;

clang/lib/Sema/SemaDecl.cpp

@@ -4766,7 +4766,10 @@ Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS, DeclSpec &DS,
   if (!DeclaresAnything) {
     // In C, we allow this as a (popular) extension / bug. Don't bother
     // producing further diagnostics for redundant qualifiers after this.
-    Diag(DS.getBeginLoc(), diag::ext_no_declarators) << DS.getSourceRange();
+    Diag(DS.getBeginLoc(), (IsExplicitInstantiation || !TemplateParams.empty())
+                               ? diag::err_no_declarators
+                               : diag::ext_no_declarators)
+        << DS.getSourceRange();
     return TagD;
   }
 

clang/test/Parser/cxx-template-decl.cpp

@@ -12,7 +12,7 @@ export template x;      // expected-error {{expected '<' after 'template'}}
 export template<class T> class x0; // expected-warning {{exported templates are unsupported}}
 template < ;            // expected-error {{expected template parameter}} \
 // expected-error{{expected ',' or '>' in template-parameter-list}} \
-// expected-warning {{declaration does not declare anything}}
+// expected-error {{declaration does not declare anything}}
 template <int +> struct x1; // expected-error {{expected ',' or '>' in template-parameter-list}}
 
 // verifies that we only walk to the ',' & still produce errors on the rest of the template parameters
@@ -286,3 +286,12 @@ namespace PR45239 {
   template<int> int b;
   template<int> auto f() -> b<0>; // expected-error +{{}}
 }
+
+namespace PR46231 {
+  template; // expected-error {{declaration does not declare anything}}
+  template<>; // expected-error {{declaration does not declare anything}}
+  template<int>; // expected-error {{declaration does not declare anything}}
+  template int; // expected-error {{declaration does not declare anything}}
+  template<> int; // expected-error {{declaration does not declare anything}}
+  template<int> int; // expected-error {{declaration does not declare anything}}
+}

clang/test/SemaCXX/PR16677.cpp

@@ -11,5 +11,5 @@ template<class T,  // Should be angle bracket instead of comma
 class Derived : public Base<T> { // expected-error{{'Derived' cannot be defined in a type specifier}}
   Class_With_Destructor member;
 }; // expected-error{{expected ',' or '>' in template-parameter-list}}
-   // expected-warning@-1{{declaration does not declare anything}}
+   // expected-error@-1{{declaration does not declare anything}}
 

clang/test/SemaCXX/invalid-template-params.cpp

@@ -5,7 +5,7 @@ template<class> class Foo {
                       // expected-note@-1 {{'UBar' declared here}}
   void foo1(); // expected-error {{a non-type template parameter cannot have type 'class UBar'}}
                // expected-error@-1 {{expected ',' or '>' in template-parameter-list}}
-               // expected-warning@-2 {{declaration does not declare anything}}
+               // expected-error@-2 {{declaration does not declare anything}}
 };
 
 Foo<int>::UBar g1; // expected-error {{no type named 'UBar' in 'Foo<int>'}}
@@ -16,7 +16,7 @@ public:
   struct S0 {}; // expected-error {{'S0' cannot be defined in a type specifier}}
                 // expected-error@-1 {{cannot combine with previous 'type-name' declaration specifier}}
                 // expected-error@-2 {{expected ',' or '>' in template-parameter-list}}
-                // expected-warning@-3 {{declaration does not declare anything}}
+                // expected-error@-3 {{declaration does not declare anything}}
   C0() : m(new S0<int>) {} // expected-error {{expected '(' for function-style cast or type construction}}
                            // expected-error@-1 {{expected expression}}
   S0<int> *m; // expected-error {{expected member name or ';' after declaration specifiers}}

clang/test/SemaTemplate/template-decl-fail.cpp

@@ -4,7 +4,7 @@ template<typename T> typedef T X; // expected-error{{typedef cannot be a templat
 
 template<typename T>
 enum t0 { A = T::x }; // expected-error{{enumeration cannot be a template}} \
-                      // expected-warning{{declaration does not declare anything}}
+                      // expected-error{{declaration does not declare anything}}
 
 enum e0 {};
 template<int x> enum e0 f0(int a=x) {}