Template Diagnostic Improvements (llvm#99933)

It turns out `SemaTemplate` handles this type of diagnostic already,
however when template gets encountered, it never gets parsed as a
possible statement or declaration, only as an expression.

Fixes llvm#17959.

Author: bradenhelmer

clang/docs/ReleaseNotes.rst

@@ -110,6 +110,16 @@ Attribute Changes in Clang
 Improvements to Clang's diagnostics
 -----------------------------------
 
+- Some template related diagnostics have been improved.
+
+  .. code-block:: c++
+    
+     void foo() { template <typename> int i; } // error: templates can only be declared in namespace or class scope
+
+     struct S {
+      template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
+     };
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5168,7 +5168,7 @@ def warn_cxx11_compat_variable_template : Warning<
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def err_template_variable_noparams : Error<
   "extraneous 'template<>' in declaration of variable %0">;
-def err_template_member : Error<"member %0 declared as a template">;
+def err_template_member : Error<"non-static data member %0 cannot be declared as a template">;
 def err_member_with_template_arguments : Error<"member %0 cannot have template arguments">;
 def err_template_member_noparams : Error<
   "extraneous 'template<>' in declaration of member %0">;

clang/lib/Parse/ParseStmt.cpp

@@ -299,6 +299,15 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
     goto Retry;
   }
 
+  case tok::kw_template: {
+    SourceLocation DeclEnd;
+    ParsedAttributes Attrs(AttrFactory);
+    ParseTemplateDeclarationOrSpecialization(DeclaratorContext::Block, DeclEnd,
+                                             Attrs,
+                                             getAccessSpecifierIfPresent());
+    return StmtError();
+  }
+
   case tok::kw_case:                // C99 6.8.1: labeled-statement
     return ParseCaseStatement(StmtCtx);
   case tok::kw_default:             // C99 6.8.1: labeled-statement

clang/test/CXX/drs/cwg6xx.cpp

@@ -1265,7 +1265,7 @@ namespace cwg687 { // cwg687 (9 c++20, but the issue is still considered open)
 
     // This is not.
     template g<int>(a);
-    // expected-error@-1 {{expected expression}}
+    // expected-error@-1 {{expected '<' after 'template'}}
   }
 }
 

clang/test/CXX/temp/temp.spec/temp.expl.spec/p2-20.cpp

@@ -56,7 +56,7 @@ struct B {
   static int y;
 
   template<typename T>
-  int z; // expected-error {{member 'z' declared as a template}}
+  int z; // expected-error {{non-static data member 'z' cannot be declared as a template}}
 
   template<typename T>
   static int x<T*>;
@@ -65,7 +65,7 @@ struct B {
   static int y<T*>;
 
   template<typename T>
-  int x<T**>; // expected-error {{member 'x' declared as a template}}
+  int x<T**>; // expected-error {{non-static data member 'x' cannot be declared as a template}}
 
   template<>
   int x<short>;
@@ -169,7 +169,7 @@ struct D {
   static int y;
 
   template<typename U>
-  int z; // expected-error {{member 'z' declared as a template}}
+  int z; // expected-error {{non-static data member 'z' cannot be declared as a template}}
 
   template<typename U>
   static int x<U*>;
@@ -178,7 +178,7 @@ struct D {
   static int y<U*>;
 
   template<typename U>
-  int x<U**>; // expected-error {{member 'x' declared as a template}}
+  int x<U**>; // expected-error {{non-static data member 'x' cannot be declared as a template}}
 
   template<>
   int x<short>;

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

@@ -297,3 +297,7 @@ namespace PR46231 {
   template<> int; // expected-error {{declaration does not declare anything}}
   template<int> int; // expected-error {{declaration does not declare anything}}
 }
+
+namespace PR99933 {
+  void foo() { template <typename> int i; } // expected-error {{templates can only be declared in namespace or class scope}}
+}

clang/test/SemaCXX/cxx1y-variable-templates_in_class.cpp

@@ -9,12 +9,12 @@
 #endif
 
 class A {
-  template<typename T> CONST T wrong;           // expected-error {{member 'wrong' declared as a template}}
-  template<typename T> CONST T wrong_init = 5;      // expected-error {{member 'wrong_init' declared as a template}}
+  template<typename T> CONST T wrong;           // expected-error {{non-static data member 'wrong' cannot be declared as a template}}
+  template<typename T> CONST T wrong_init = 5;      // expected-error {{non-static data member 'wrong_init' cannot be declared as a template}}
   template<typename T, typename T0> static CONST T right = T(100);
   template<typename T> static CONST T right<T,int> = 5;
-  template<typename T> CONST int right<int,T>;  // expected-error {{member 'right' declared as a template}}
-  template<typename T> CONST float right<float,T> = 5;  // expected-error {{member 'right' declared as a template}}
+  template<typename T> CONST int right<int,T>;  // expected-error {{non-static data member 'right' cannot be declared as a template}}
+  template<typename T> CONST float right<float,T> = 5;  // expected-error {{non-static data member 'right' cannot be declared as a template}}
 #ifdef PRECXX11
                                                         // expected-warning@-2 {{in-class initializer for static data member of type 'const float' is a GNU extension}}
 #else
@@ -161,14 +161,14 @@ namespace non_const_init {
 #ifndef PRECXX11
 namespace constexpred {
   class A {
-    template<typename T> constexpr T wrong;           // expected-error {{member 'wrong' declared as a template}}
+    template<typename T> constexpr T wrong;           // expected-error {{non-static data member 'wrong' cannot be declared as a template}}
                                                       // expected-error@-1 {{declaration of constexpr static data member 'wrong' requires an initializer}}
-    template<typename T> constexpr T wrong_init = 5;  // expected-error {{member 'wrong_init' declared as a template}}
+    template<typename T> constexpr T wrong_init = 5;  // expected-error {{non-static data member 'wrong_init' cannot be declared as a template}}
     template<typename T, typename T0> static constexpr T right = T(100);
     template<typename T> static constexpr T right<T,int> = 5;
-    template<typename T> constexpr int right<int,T>;         // expected-error {{member 'right' declared as a template}}
+    template<typename T> constexpr int right<int,T>;         // expected-error {{non-static data member 'right' cannot be declared as a template}}
                                                              // expected-error@-1 {{declaration of constexpr static data member 'right<int, T>' requires an initializer}}
-    template<typename T> constexpr float right<float,T> = 5; // expected-error {{member 'right' declared as a template}}
+    template<typename T> constexpr float right<float,T> = 5; // expected-error {{non-static data member 'right' cannot be declared as a template}}
     template<> constexpr int right<int,int> = 7;
     template<> constexpr float right<float, int>; // expected-error {{declaration of constexpr static data member 'right<float, int>' requires an initializer}}
     template static constexpr int right<int,int>;     // expected-error {{expected '<' after 'template'}}

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

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only
+// PR99933
+
+struct S {
+  template <typename> int i; // expected-error {{non-static data member 'i' cannot be declared as a template}}
+};

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

@@ -63,7 +63,7 @@ class X {
 };
 
 void f() {
-  template<typename T> class X; // expected-error{{expression}}
+  template<typename T> class X; // expected-error{{templates can only be declared in namespace or class scope}}
 }
 
 template<typename T> class X1 var; // expected-error {{variable has incomplete type 'class X1'}} \

clang/test/SemaTemplate/nested-template.cpp

@@ -132,7 +132,7 @@ namespace PR10896 {
   private:
 
     template<typename T>
-    T SomeField; // expected-error {{member 'SomeField' declared as a template}}
+    T SomeField; // expected-error {{non-static data member 'SomeField' cannot be declared as a template}}
     template<> int SomeField2; // expected-error {{extraneous 'template<>' in declaration of variable 'SomeField2'}}
   };