[clang] require template arg list after template kw (#80801)

Require a template argument list after an identifier prefixed by the
template keyword. Introduced by [CWG
96](https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#96),
but the current wording of
[[temp.names]p5](https://eel.is/c++draft/temp.names#6) was introduced in
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html),
and became [temp.names]p6 somewhere else.

Fixes #53095

---------

Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>

Author: evelez7

clang/docs/ReleaseNotes.rst

@@ -251,6 +251,9 @@ Resolutions to C++ Defect Reports
 - P0522 implementation is enabled by default in all language versions, and
   provisional wording for CWG2398 is implemented.
 
+- Clang now requires a template argument list after a template keyword.
+  (`CWG96: Syntactic disambiguation using the template keyword <https://cplusplus.github.io/CWG/issues/96.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -887,6 +887,10 @@ def err_requires_expr_in_simple_requirement : Error<
   "requires expression in requirement body; did "
   "you intend to place it in a nested requirement? (add another 'requires' "
   "before the expression)">;
+def missing_template_arg_list_after_template_kw : Extension<
+  "a template argument list is expected after a name prefixed by the template "
+  "keyword">, InGroup<DiagGroup<"missing-template-arg-list-after-template-kw">>,
+  DefaultError;
 
 def err_missing_dependent_template_keyword : Error<
   "use 'template' keyword to treat '%0' as a dependent template name">;

clang/lib/Parse/ParseExprCXX.cpp

@@ -14,6 +14,7 @@
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/Basic/PrettyStackTrace.h"
+#include "clang/Basic/TemplateKinds.h"
 #include "clang/Basic/TokenKinds.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Parse/ParseDiagnostic.h"
@@ -3026,13 +3027,23 @@ bool Parser::ParseUnqualifiedId(CXXScopeSpec &SS, ParsedType ObjectType,
           SS, ObjectType, ObjectHadErrors,
           TemplateKWLoc ? *TemplateKWLoc : SourceLocation(), Id, IdLoc,
           EnteringContext, Result, TemplateSpecified);
-    else if (TemplateSpecified &&
-             Actions.ActOnTemplateName(
-                 getCurScope(), SS, *TemplateKWLoc, Result, ObjectType,
-                 EnteringContext, Template,
-                 /*AllowInjectedClassName*/ true) == TNK_Non_template)
-      return true;
 
+    if (TemplateSpecified) {
+      TemplateNameKind TNK =
+          Actions.ActOnTemplateName(getCurScope(), SS, *TemplateKWLoc, Result,
+                                    ObjectType, EnteringContext, Template,
+                                    /*AllowInjectedClassName=*/true);
+      if (TNK == TNK_Non_template)
+        return true;
+
+      // C++2c [tem.names]p6
+      // A name prefixed by the keyword template shall be followed by a template
+      // argument list or refer to a class template or an alias template.
+      if ((TNK == TNK_Function_template || TNK == TNK_Dependent_template_name ||
+           TNK == TNK_Var_template) &&
+          !Tok.is(tok::less))
+        Diag(IdLoc, diag::missing_template_arg_list_after_template_kw);
+    }
     return false;
   }
 

clang/test/CXX/drs/cwg0xx.cpp

@@ -1404,7 +1404,7 @@ namespace cwg95 { // cwg95: 3.3
   //   expected-note@#cwg95-C-f {{implicitly declared private here}}
 }
 
-namespace cwg96 { // cwg96: no
+namespace cwg96 { // cwg96: sup P1787
   struct A {
     void f(int);
     template<typename T> int f(T);
@@ -1419,6 +1419,7 @@ namespace cwg96 { // cwg96: no
     // name a class template.
     // FIXME: What about alias templates?
     int k2 = a.template f(1);
+    // expected-error@-1 {{a template argument list is expected after a name prefixed by the template keyword}}
     A::template S<int> s;
     B<A::template S> b;
   }

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

@@ -384,16 +384,16 @@ namespace dependent_static_var_template {
   struct A {
     template<int = 0> static int n; // expected-note 2{{here}}
   };
-  int &r = A::template n; // expected-error {{use of variable template 'A::template n' requires template arguments}}
+  int &r = A::template n; // expected-error {{use of variable template 'A::template n' requires template arguments}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
   template<typename T>
-  int &f() { return T::template n; } // expected-error {{use of variable template 'A::template n' requires template arguments}}
+  int &f() { return T::template n; } // expected-error {{use of variable template 'A::template n' requires template arguments}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
   int &s = f<A>(); // expected-note {{instantiation of}}
 
   namespace B {
     template<int = 0> static int n; // expected-note {{here}}
   }
-  int &t = B::template n; // expected-error {{use of variable template 'B::template n' requires template arguments}}
+  int &t = B::template n; // expected-error {{use of variable template 'B::template n' requires template arguments}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
   struct C {
     template <class T> static T G;

clang/test/SemaCXX/template-specialization.cpp

@@ -11,7 +11,7 @@ struct B {
   template <int i>
   static void foo() {
     int array[i];
-    A::template bar(array[0]); // expected-error {{no matching function for call to 'bar'}}
+    A::template bar(array[0]); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}} expected-error {{no matching function for call to 'bar'}}
   }
 };
 

clang/test/SemaTemplate/dependent-names.cpp

@@ -420,7 +420,7 @@ template <typename> struct CT2 {
 template <typename T> int CT2<int>::X<>; // expected-error {{template parameter list matching the non-templated nested type 'CT2<int>' should be empty}}
 
 namespace DependentTemplateIdWithNoArgs {
-  template<typename T> void f() { T::template f(); }
+  template<typename T> void f() { T::template f(); } // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
   struct X {
     template<int = 0> static void f();
   };
@@ -431,7 +431,7 @@ namespace DependentUnresolvedUsingTemplate {
   template<typename T>
   struct X : T {
     using T::foo;
-    void f() { this->template foo(); } // expected-error {{does not refer to a template}}
+    void f() { this->template foo(); } // expected-error {{does not refer to a template}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
     void g() { this->template foo<>(); } // expected-error {{does not refer to a template}}
     void h() { this->template foo<int>(); } // expected-error {{does not refer to a template}}
   };
@@ -450,7 +450,7 @@ namespace DependentUnresolvedUsingTemplate {
 namespace PR37680 {
   template <class a> struct b : a {
     using a::add;
-    template<int> int add() { return this->template add(0); }
+    template<int> int add() { return this->template add(0); } // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
   };
   struct a {
     template<typename T = void> int add(...);

clang/test/SemaTemplate/template-id-expr.cpp

@@ -65,23 +65,23 @@ struct Y0 {
   template<typename U>
   void f() {
     Y0::template f1<U>(0);
-    Y0::template f1(0);
-    this->template f1(0);
+    Y0::template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
+    this->template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     Y0::template f2<U>(0);
-    Y0::template f2(0);
+    Y0::template f2(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     Y0::template f3(0); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
     Y0::template f3(); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
 
     int x;
     x = Y0::f4(0);
     x = Y0::f4<int>(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
-    x = Y0::template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = Y0::template f4(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}} expected-error {{assigning to 'int' from incompatible type 'void'}}
 
     x = this->f4(0);
     x = this->f4<int>(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
-    x = this->template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = this->template f4(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}} expected-error {{assigning to 'int' from incompatible type 'void'}}
   }
 };
 
@@ -109,23 +109,23 @@ struct Y1 {
   template<typename U>
   void f() {
     Y1::template f1<U>(0);
-    Y1::template f1(0);
-    this->template f1(0);
+    Y1::template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
+    this->template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     Y1::template f2<U>(0);
-    Y1::template f2(0);
+    Y1::template f2(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     Y1::template f3(0); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
     Y1::template f3(); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
 
     int x;
     x = Y1::f4(0);
     x = Y1::f4<int>(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
-    x = Y1::template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = Y1::template f4(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}} expected-error {{assigning to 'int' from incompatible type 'void'}}
 
     x = this->f4(0);
     x = this->f4<int>(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
-    x = this->template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = this->template f4(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}} expected-error {{assigning to 'int' from incompatible type 'void'}}
   }
 };
 
@@ -138,23 +138,23 @@ struct Y2 : Y1<T> {
   template<typename U>
   void f(Y1 *p) {
     Y1::template f1<U>(0);
-    Y1::template f1(0);
-    p->template f1(0);
+    Y1::template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
+    p->template f1(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     Y1::template f2<U>(0);
-    Y1::template f2(0);
+    Y1::template f2(0); // expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
-    Y1::template f3(0); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
-    Y1::template f3(); // expected-error {{'f3' following the 'template' keyword does not refer to a template}}
+    Y1::template f3(0); // expected-error {{'f3' following the 'template' keyword does not refer to a template}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
+    Y1::template f3(); // expected-error {{'f3' following the 'template' keyword does not refer to a template}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     int x;
     x = Y1::f4(0);
     x = Y1::f4<int>(0); // expected-error {{use 'template'}} expected-error {{assigning to 'int' from incompatible type 'void'}}
-    x = Y1::template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = Y1::template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 
     x = p->f4(0);
     x = p->f4<int>(0); // expected-error {{assigning to 'int' from incompatible type 'void'}} expected-error {{use 'template'}}
-    x = p->template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}}
+    x = p->template f4(0); // expected-error {{assigning to 'int' from incompatible type 'void'}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
   }
 };
 
@@ -169,7 +169,7 @@ struct A {
 
 template<int I>
 void f5() {
-  A::template B<I>::template b1(); // expected-error {{'b1' following the 'template' keyword does not refer to a template}}
+  A::template B<I>::template b1(); // expected-error {{'b1' following the 'template' keyword does not refer to a template}} expected-error {{a template argument list is expected after a name prefixed by the template keyword}}
 }
 
 template void f5<0>(); // expected-note {{in instantiation of function template specialization 'f5<0>' requested here}}

clang/test/SemaTemplate/template-id-printing.cpp

@@ -99,8 +99,6 @@ template <typename T>
 void test() {
   // CHECK: S<T>::foo;
   S<T>::foo;
-  // CHECK: S<T>::template foo;
-  S<T>::template foo;
   // CHECK: S<T>::template foo<>;
   S<T>::template foo<>;
   // CHECK: S<T>::template foo<T>;
@@ -121,21 +119,10 @@ void test() {
   S<T> s;
   // CHECK: s.foo;
   s.foo;
-  // CHECK: s.template foo;
-  s.template foo;
   // CHECK: s.template foo<>;
   s.template foo<>;
   // CHECK: s.template foo<T>;
   s.template foo<T>;
 }
 
 } // namespace DSME
-
-namespace DSDRE_withImplicitTemplateArgs {
-
-template <typename T> void foo() {
-  // CHECK: T::template bar();
-  T::template bar();
-}
-
-} // namespace DSDRE_withImplicitTemplateArgs

clang/www/cxx_dr_status.html

@@ -614,7 +614,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/96.html">96</a></td>
     <td>C++11</td>
     <td>Syntactic disambiguation using the <TT>template</TT> keyword</td>
-    <td class="none" align="center">No</td>
+    <td class="na" align="center">Superseded by <a href="https://wg21.link/P1787">P1787</a></td>
   </tr>
   <tr id="97">
     <td><a href="https://cplusplus.github.io/CWG/issues/97.html">97</a></td>