c++: ICE with bogus late return type [PR99803]

Here we ICE when compiling this code in C++20, because we're trying to
slam a 'typename' after the ->.  The cp_parser_template_id call just
before the spot I'm changing parsed A::template A<int> as a BASELINK
that contains a constructor, but make_typename_type crashes on that.

This patch makes make_typename_type more robust instead of checking
for is_overloaded_fn prior calling it.

gcc/cp/ChangeLog:

	PR c++/99803
	* decl.c (make_typename_type): Give an error and return when
	name is is_overloaded_fn.
	* parser.c (cp_parser_class_name): Don't check is_overloaded_fn
	before calling make_typename_type.

gcc/testsuite/ChangeLog:

	PR c++/99803
	* g++.dg/cpp2a/typename14.C: Don't expect particular error
	messages.
	* g++.dg/cpp2a/typename19.C: New test.

gcc/cp/decl.c

@@ -4055,6 +4055,12 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
 	error ("%qD used without template arguments", name);
       return error_mark_node;
     }
+  else if (is_overloaded_fn (name))
+    {
+      if (complain & tf_error)
+	error ("%qD is a function, not a type", name);
+      return error_mark_node;
+    }
   gcc_assert (identifier_p (name));
   gcc_assert (TYPE_P (context));
 
@@ -4066,7 +4072,7 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
 	error ("%q#T is not a class", context);
       return error_mark_node;
     }
-  
+
   /* When the CONTEXT is a dependent type,  NAME could refer to a
      dependent base class of CONTEXT.  But look inside it anyway
      if CONTEXT is a currently open scope, in case it refers to a

gcc/cp/parser.c

@@ -24730,9 +24730,7 @@ cp_parser_class_name (cp_parser *parser,
   decl = cp_parser_maybe_treat_template_as_class (decl, class_head_p);
 
   /* If this is a typename, create a TYPENAME_TYPE.  */
-  if (typename_p
-      && decl != error_mark_node
-      && !is_overloaded_fn (decl))
+  if (typename_p && decl != error_mark_node)
     {
       decl = make_typename_type (scope, decl, typename_type,
 				 /*complain=*/tf_error);

gcc/testsuite/g++.dg/cpp2a/typename14.C

@@ -8,7 +8,7 @@ template<typename> struct A
 
 template<typename T>
 template<typename U>
-A<T>::A<U> () // { dg-error "partial specialization" }
+A<T>::A<U> () // { dg-error "" }
 {
 }
 
@@ -19,7 +19,7 @@ template<typename> struct B
 
 template<typename T>
 template<typename U>
-B<T>::foo<int>(int) // { dg-error "partial specialization|declaration" }
+B<T>::foo<int>(int) // { dg-error "" }
 {
   return 1;
 }

gcc/testsuite/g++.dg/cpp2a/typename19.C

@@ -0,0 +1,5 @@
+// PR c++/99803
+// { dg-do compile { target c++20 } }
+
+struct A { template<typename T> A(T); };
+auto A(unsigned) -> A::template A<int>; // { dg-error "not a type" }