c++: DR1710, template keyword in a typename-specifier [PR94057]

Consider

  template <typename T> class A {
    template <typename U> class B {
      void fn(typename A<T>::B<U>);
    };
  };

which is rejected with
error: 'typename A<T>::B' names 'template<class T> template<class U> class A<T>::B', which is not a type
whereas clang/icc/msvc accept it.

"typename A<T>::B<U>" is a typename-specifier.  Sadly, our comments
don't mention it anywhere, because the typename-specifier wasn't in C++11;
it was only added to the language in N1376.  Instead, we handle it as
an elaborated-type-specifier (not a problem thus far).   So we get to
cp_parser_nested_name_specifier_opt which has a loop that breaks if we
don't see a < or ::, but that means we can -- tentatively -- parse even
B<U> which is not a nested-name-specifier (it doesn't end with a ::).

I think this should compile because [temp.names]/4 says: "In a qualified-id
used as the name in a typename-specifier, elaborated-type-specifier,
using-declaration, or class-or-decltype, an optional keyword template
appearing at the top level is ignored.", added in DR 1710.  Also see
DR 1812.

This issue on its own is not a significant problem or a regression.
However, in C++20, the typename here becomes optional, and so this test
is rejected in C++20, but accepted in C++17:

  template <typename T> class A {
    template <typename U> class B {
      void fn(A<T>::B<U>);
    };
  };

Here we morph A<T>::B<U> into a typename-specifier, but that happens
in cp_parser_simple_type_specifier and we never handle it as above.
To fake the template keyword I'm afraid we need to use cp_parser_template_id
with template_keyword_p=true as in the patch below.  The tricky thing
is to avoid breaking concepts.

To handle DR 1710, I made cp_parser_nested_name_specifier_opt assume that
when we're naming a type, the template keyword is present, too.  That
revealed a bug: DR 1710 also says that the template keyword can be followed
by an alias template, but we weren't prepared to handle that.  alias-decl?.C
exercise this.

gcc/cp:

	DR 1710
	PR c++/94057 - template keyword in a typename-specifier.
	* parser.c (check_template_keyword_in_nested_name_spec): New.
	(cp_parser_nested_name_specifier_opt): Implement DR1710, optional
	'template'.  Call check_template_keyword_in_nested_name_spec.
	(cp_parser_simple_type_specifier): Assume that a <
	following a qualified-id in a typename-specifier begins
	a template argument list.

gcc/testsuite:

	DR 1710
	PR c++/94057 - template keyword in a typename-specifier.
	* g++.dg/cpp1y/alias-decl1.C: New test.
	* g++.dg/cpp1y/alias-decl2.C: New test.
	* g++.dg/cpp1y/alias-decl3.C: New test.
	* g++.dg/parse/missing-template1.C: Update dg-error.
	* g++.dg/parse/template3.C: Likewise.
	* g++.dg/template/error4.C: Likewise.
	* g++.dg/template/meminit2.C: Likewise.
	* g++.dg/template/dependent-name5.C: Likewise.
	* g++.dg/template/dependent-name7.C: New test.
	* g++.dg/template/dependent-name8.C: New test.
	* g++.dg/template/dependent-name9.C: New test.
	* g++.dg/template/dependent-name10.C: New test.
	* g++.dg/template/dependent-name11.C: New test.
	* g++.dg/template/dependent-name12.C: New test.
	* g++.dg/template/dependent-name13.C: New test.
	* g++.dg/template/dr1794.C: New test.
	* g++.dg/template/dr314.C: New test.
	* g++.dg/template/dr1710.C: New test.
	* g++.dg/template/dr1710-2.C: New test.
	* g++.old-deja/g++.pt/crash38.C: Update dg-error.

gcc/cp/ChangeLog

@@ -1,3 +1,14 @@
+2020-03-26  Marek Polacek  <polacek@redhat.com>
+
+	DR 1710
+	PR c++/94057 - template keyword in a typename-specifier.
+	* parser.c (check_template_keyword_in_nested_name_spec): New.
+	(cp_parser_nested_name_specifier_opt): Implement DR1710, optional
+	'template'.  Call check_template_keyword_in_nested_name_spec.
+	(cp_parser_simple_type_specifier): Assume that a <
+	following a qualified-id in a typename-specifier begins
+	a template argument list.
+
 2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* coroutines.cc (coro_init_identifiers): Initialize an identifier

gcc/cp/parser.c

@@ -6266,6 +6266,32 @@ cp_parser_unqualified_id (cp_parser* parser,
     }
 }
 
+/* Check [temp.names]/5: A name prefixed by the keyword template shall
+   be a template-id or the name shall refer to a class template or an
+   alias template.  */
+
+static void
+check_template_keyword_in_nested_name_spec (tree name)
+{
+  if (CLASS_TYPE_P (name)
+      && ((CLASSTYPE_USE_TEMPLATE (name)
+	   && PRIMARY_TEMPLATE_P (CLASSTYPE_TI_TEMPLATE (name)))
+	  || CLASSTYPE_IS_TEMPLATE (name)))
+    return;
+
+  if (TREE_CODE (name) == TYPENAME_TYPE
+      && TREE_CODE (TYPENAME_TYPE_FULLNAME (name)) == TEMPLATE_ID_EXPR)
+    return;
+  /* Alias templates are also OK.  */
+  else if (alias_template_specialization_p (name, nt_opaque))
+    return;
+
+  permerror (input_location, TYPE_P (name)
+	     ? G_("%qT is not a template")
+	     : G_("%qD is not a template"),
+	     name);
+}
+
 /* Parse an (optional) nested-name-specifier.
 
    nested-name-specifier: [C++98]
@@ -6389,7 +6415,17 @@ cp_parser_nested_name_specifier_opt (cp_parser *parser,
       /* Look for the optional `template' keyword, if this isn't the
 	 first time through the loop.  */
       if (success)
-	template_keyword_p = cp_parser_optional_template_keyword (parser);
+	{
+	  template_keyword_p = cp_parser_optional_template_keyword (parser);
+	  /* DR1710: "In a qualified-id used as the name in
+	     a typename-specifier, elaborated-type-specifier, using-declaration,
+	     or class-or-decltype, an optional keyword template appearing at
+	     the top level is ignored."  */
+	  if (!template_keyword_p
+	      && typename_keyword_p
+	      && cp_parser_nth_token_starts_template_argument_list_p (parser, 2))
+	    template_keyword_p = true;
+	}
 
       /* Save the old scope since the name lookup we are about to do
 	 might destroy it.  */
@@ -6580,18 +6616,8 @@ cp_parser_nested_name_specifier_opt (cp_parser *parser,
       if (TREE_CODE (new_scope) == TYPE_DECL)
 	new_scope = TREE_TYPE (new_scope);
       /* Uses of "template" must be followed by actual templates.  */
-      if (template_keyword_p
-	  && !(CLASS_TYPE_P (new_scope)
-	       && ((CLASSTYPE_USE_TEMPLATE (new_scope)
-		    && PRIMARY_TEMPLATE_P (CLASSTYPE_TI_TEMPLATE (new_scope)))
-		   || CLASSTYPE_IS_TEMPLATE (new_scope)))
-	  && !(TREE_CODE (new_scope) == TYPENAME_TYPE
-	       && (TREE_CODE (TYPENAME_TYPE_FULLNAME (new_scope))
-		   == TEMPLATE_ID_EXPR)))
-	permerror (input_location, TYPE_P (new_scope)
-		   ? G_("%qT is not a template")
-		   : G_("%qD is not a template"),
-		   new_scope);
+      if (template_keyword_p)
+	check_template_keyword_in_nested_name_spec (new_scope);
       /* If it is a class scope, try to complete it; we are about to
 	 be looking up names inside the class.  */
       if (TYPE_P (new_scope)
@@ -18120,6 +18146,33 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 		}
 	    }
 	}
+      /* DR 1812: A < following a qualified-id in a typename-specifier
+	 could safely be assumed to begin a template argument list, so
+	 the template keyword should be optional.  */
+      else if (parser->scope
+	       && qualified_p
+	       && typename_p
+	       && cp_lexer_next_token_is (parser->lexer, CPP_TEMPLATE_ID))
+	{
+	  cp_parser_parse_tentatively (parser);
+
+	  type = cp_parser_template_id (parser,
+					/*template_keyword_p=*/true,
+					/*check_dependency_p=*/true,
+					none_type,
+					/*is_declaration=*/false);
+	  /* This is handled below, so back off.  */
+	  if (type && concept_check_p (type))
+	    cp_parser_simulate_error (parser);
+
+	  if (!cp_parser_parse_definitely (parser))
+	    type = NULL_TREE;
+	  else if (TREE_CODE (type) == TEMPLATE_ID_EXPR)
+	    type = make_typename_type (parser->scope, type, typename_type,
+				       /*complain=*/tf_error);
+	  else if (TREE_CODE (type) != TYPE_DECL)
+	    type = NULL_TREE;
+	}
 
       /* Otherwise, look for a type-name.  */
       if (!type)

gcc/testsuite/ChangeLog

@@ -1,3 +1,28 @@
+2020-03-26  Marek Polacek  <polacek@redhat.com>
+
+	DR 1710
+	PR c++/94057 - template keyword in a typename-specifier.
+	* g++.dg/cpp1y/alias-decl1.C: New test.
+	* g++.dg/cpp1y/alias-decl2.C: New test.
+	* g++.dg/cpp1y/alias-decl3.C: New test.
+	* g++.dg/parse/missing-template1.C: Update dg-error.
+	* g++.dg/parse/template3.C: Likewise.
+	* g++.dg/template/error4.C: Likewise.
+	* g++.dg/template/meminit2.C: Likewise.
+	* g++.dg/template/dependent-name5.C: Likewise.
+	* g++.dg/template/dependent-name7.C: New test.
+	* g++.dg/template/dependent-name8.C: New test.
+	* g++.dg/template/dependent-name9.C: New test.
+	* g++.dg/template/dependent-name10.C: New test.
+	* g++.dg/template/dependent-name11.C: New test.
+	* g++.dg/template/dependent-name12.C: New test.
+	* g++.dg/template/dependent-name13.C: New test.
+	* g++.dg/template/dr1794.C: New test.
+	* g++.dg/template/dr314.C: New test.
+	* g++.dg/template/dr1710.C: New test.
+	* g++.dg/template/dr1710-2.C: New test.
+	* g++.old-deja/g++.pt/crash38.C: Update dg-error.
+
 2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* g++.dg/coroutines/torture/co-ret-09-bool-await-susp.C: Amend

gcc/testsuite/g++.dg/cpp1y/alias-decl1.C

@@ -0,0 +1,9 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+// { dg-do compile { target c++14 } }
+
+template <int> struct S {
+  template <int> struct A;
+  template <int N> using U = typename A<N>::foo;
+};
+template <typename T> typename S<1>::U<T::foo>::type a;
+template <typename T> typename S<1>::template U<T::foo>::type a2;

gcc/testsuite/g++.dg/cpp1y/alias-decl2.C

@@ -0,0 +1,8 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+// { dg-do compile { target c++14 } }
+
+template <typename T> struct S {
+  template <typename TT>
+  using U = TT;
+};
+template <typename T> typename S<int>::template U<T>::type foo;

gcc/testsuite/g++.dg/cpp1y/alias-decl3.C

@@ -0,0 +1,9 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+// { dg-do compile { target c++14 } }
+
+template <typename T> struct S {
+  template<typename TT> struct A { };
+  typedef A<int> type;
+};
+template <typename T> typename S<int>::template A<int> foo;
+template <typename T> typename S<int>::template type foo2; // { dg-error "expected template-id" }

gcc/testsuite/g++.dg/parse/missing-template1.C

@@ -12,9 +12,7 @@ template <typename T> struct A
 
 template <typename T> void foo()
 {
-    typedef typename A<T>::B<T>::X Y; // { dg-error "non-template" "non" }
-    // { dg-error "not declare" "decl" { target *-*-* } .-1 }
-    // { dg-message "note" "note" { target *-*-* } .-2 }
+    typedef typename A<T>::B<T>::X Y;
 }
 
 void bar()

gcc/testsuite/g++.dg/parse/template3.C

@@ -13,7 +13,4 @@ struct X : Outer<b>::template Inner<T>
 {};
 
 template <bool b, typename T>
-struct Y : Outer<b>::Inner<T> {}; // { dg-error "used as template" "temp" }
-// { dg-error "expected" "exp" { target *-*-* } .-1 }
-// { dg-message "note" "note" { target *-*-* } .-2 }
-
+struct Y : Outer<b>::Inner<T> {};

gcc/testsuite/g++.dg/template/dependent-name10.C

@@ -0,0 +1,18 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+// { dg-do compile { target c++11 } }
+
+template<typename T> struct A {
+  template<typename U> struct B {
+    typedef int TT;
+    typedef int TT2;
+    typedef int TT3;
+    typedef int TT4;
+  };
+};
+
+struct X : A<int>::B<int> {
+  using A<int>::template B<int>::TT;
+  using typename A<int>::template B<int>::TT2;
+  using A<int>::B<int>::TT3;
+  using typename A<int>::B<int>::TT4;
+};

gcc/testsuite/g++.dg/template/dependent-name11.C

@@ -0,0 +1,15 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+// { dg-do compile { target c++11 } }
+
+template<typename T> struct A {
+  template<typename U>
+  struct W { };
+};
+
+void
+g ()
+{
+  // class-key nested-name-specifier template[opt] simple-template-id
+  struct A<int>::W<int> w;
+  struct A<int>::template W<int> w2;
+}

gcc/testsuite/g++.dg/template/dependent-name12.C

@@ -0,0 +1,7 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+
+template <bool> struct A;
+template <typename, typename> struct B;
+template <typename T, typename U, typename V> struct B<T U::*, V> {
+  typename A<V::x>::type::type t;
+};

gcc/testsuite/g++.dg/template/dependent-name13.C

@@ -0,0 +1,8 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+
+template<typename T> struct S {
+  void fn(typename T::template B<int>::template C<int>);
+  void fn2(typename T::B<int>::template C<int>);
+  void fn3(typename T::template B<int>::C<int>);
+  void fn4(typename T::B<int>::C<int>);
+};

gcc/testsuite/g++.dg/template/dependent-name5.C

@@ -22,9 +22,7 @@ struct A
 
   typedef N<int>       type6;
   typedef A::N<int>    type7;
-// { dg-error "" "" { target c++2a } .-1 }
   typedef A<T>::N<int> type8;
-// { dg-error "" "" { target c++2a } .-1 }
   typedef A<T*>::template N<int> type9;  // { dg-error "" "" { target c++17_down } }
   typedef typename A<T*>::template N<int> type10;
 

gcc/testsuite/g++.dg/template/dependent-name7.C

@@ -0,0 +1,9 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+// { dg-do compile }
+
+template<typename T> struct A {
+  template<typename U> struct B {
+    B(A<T>::B<U>&);
+    void fn(A<T>::B<U>);
+  };
+};

gcc/testsuite/g++.dg/template/dependent-name8.C

@@ -0,0 +1,9 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+// { dg-do compile }
+
+template<typename T> struct A {
+  template<typename U> struct B {
+    B(typename A<T>::B<U>&);
+    void fn(typename A<T>::B<U>);
+  };
+};

gcc/testsuite/g++.dg/template/dependent-name9.C

@@ -0,0 +1,9 @@
+// PR c++/94057 - template keyword in a typename-specifier.
+// { dg-do compile }
+
+template<typename T> struct A {
+  template<typename U> struct B {
+    B(typename A<T>::template B<U>&);
+    void fn(typename A<T>::template B<U>);
+  };
+};

gcc/testsuite/g++.dg/template/dr1710-2.C

@@ -0,0 +1,10 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+// { dg-do compile }
+
+template<typename T> struct A {
+  template<typename U> struct B {
+  };
+};
+
+template<typename T> struct D : A<int>::B<int> {};
+template<typename T> struct D2 : A<int>::template B<int> {};

gcc/testsuite/g++.dg/template/dr1710.C

@@ -0,0 +1,9 @@
+// DR 1710 - Missing template keyword in class-or-decltype
+// { dg-do compile }
+
+template<typename T> struct D : T::template B<int>::template C<int> {};
+template<typename T> struct D2 : T::B<int>::template C<int> {};
+template<typename T> struct D3 : T::template B<int>::C<int> {};
+template<typename T> struct D4 : T::B<int>::C<int> {};
+template<typename T> struct D5 : T::template B<int>::type::type {};
+template<typename T> struct D6 : T::B<int>::type::type {};

gcc/testsuite/g++.dg/template/dr1794.C

@@ -0,0 +1,14 @@
+// DR 1794 - template keyword and alias templates.
+// { dg-do compile { target c++11 } }
+
+template<template<typename> class Template>
+struct Internal {
+  template<typename Arg>
+  using Bind = Template<Arg>;
+};
+
+template<template<typename> class Template, typename Arg>
+using Instantiate = Template<Arg>;
+
+template<template<typename> class Template, typename Argument>
+using Bind = Instantiate<Internal<Template>::template Bind, Argument>;

gcc/testsuite/g++.dg/template/dr314.C

@@ -0,0 +1,15 @@
+// DR 314 - template in base class specifier.
+
+template <typename T>
+struct A {
+  template <typename U>
+  struct B {};
+};
+
+template <typename T>
+struct C : public A<T>::template B<T> {
+};
+
+template <typename T>
+struct C2 : public A<int>::B<T> {
+};

gcc/testsuite/g++.dg/template/error4.C

@@ -5,5 +5,4 @@ template<class T> struct C1
 };
 
 template<class T, class U>
-void foo(typename C1<T>::C2<U>::Type *) { } // { dg-error "template" "error " }
-// { dg-message "note" "note" { target *-*-* } .-1 }
+void foo(typename C1<T>::C2<U>::Type *) { }

gcc/testsuite/g++.dg/template/meminit2.C

@@ -14,8 +14,6 @@ struct A : typename O<T>::template I<int> {   // { dg-error "keyword 'typename'
 
 template <typename T> 
 struct B : O<T>::template I<int> {
-  B() :    O<T>::I<int>()   // { dg-error "used as template|it is a template" }
+  B() :    O<T>::I<int>()
   {}
 };
-
-// { dg-bogus "end of input" "bogus token skipping in the parser" { xfail *-*-* } 17 }

gcc/testsuite/g++.old-deja/g++.pt/crash38.C

@@ -3,10 +3,8 @@
 
 template <class T>
 struct S {
-  typedef typename T::Y<T>::Z X; // { dg-error "non-template" "non-template" } No Y in A
-// { dg-message "note" "note" { target *-*-* } .-1 }
-// { dg-error "does not declare" "not declare" { target *-*-* } .-2 }
-  X x; // { dg-error "does not name a type" } No Y in A
+  typedef typename T::Y<T>::Z X; // { dg-error "not a class template" } No Y in A
+  X x;
 };
 
 struct A {