C++ PATCH for c++/88184, ICE when treating name as template-name

[Marek Polacek]

On Wed, Nov 28, 2018 at 10:34:11PM -0500, Jason Merrill wrote:
> On 11/28/18 10:48 AM, Marek Polacek wrote:
> > Since P0846R0 was implemented, a name will be treated as a template-name when
> > it is an unqualified-id followed by a < and name lookup finds either one or
> > more functions or finds nothing, in order to potentially cause ADL to be performed.
> > 
> > In this case, we had
> > 
> >    f<T> ();
> > 
> > where we'd found a decl for f (not a TEMPLATE_DECL) and < follows.
> 
> From the backtrace in the PR, it seems as though we're treating f<T> as
> non-dependent, which is wrong.  type_dependent_expression_p only looks at
> the arguments of a TEMPLATE_ID_EXPR if it has unknown_type_node, so we
> probably want to give it that type.

That was my first attempt but it was crashing everywhere, so I abandoned it.
But I was setting unknown_type_node in cp_parser_template_name whereas this
patch sets it in cp_parser_postfix_expression, which works and is arguably
better because it reuses the diagnostic in finish_call_expr.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-11-29  Marek Polacek  <polacek@redhat.com>

	PR c++/88184, ICE when treating name as template-name.
	* parser.c (cp_parser_postfix_expression): Set the function decl type
	to unknown type when the name-as-template-name doesn't have any
	arguments.

	* g++.dg/cpp2a/fn-template17.C: New test.
	* g++.dg/cpp2a/fn-template18.C: New test.

diff --git gcc/cp/parser.c gcc/cp/parser.c
index 3ef1eda4518..fc328c28f69 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -7241,6 +7241,20 @@ cp_parser_postfix_expression (cp_parser *parser, bool address_p, bool cast_p,
 						     complain);
 		      }
 		  }
+		else if (TREE_CODE (postfix_expression) == TEMPLATE_ID_EXPR
+			 && (TREE_CODE (TREE_OPERAND (postfix_expression, 0))
+			     == FUNCTION_DECL)
+			 && args->is_empty ())
+		  {
+		    /* We can get here when treating a name as a template name
+		       (because it was an unqualified-id followed by a < and
+		       name lookup found a decl), but now we see that there
+		       are no arguments, so ADL isn't possible.  Tweak the type
+		       so that the TEMPLATE_ID_EXPR is type-dependent and we get
+		       proper diagnostic down in finish_call_expr.  */
+		    TREE_TYPE (TREE_OPERAND (postfix_expression, 0))
+		      = unknown_type_node;
+		  }
 	      }
 
 	    if (TREE_CODE (postfix_expression) == COMPONENT_REF)
diff --git gcc/testsuite/g++.dg/cpp2a/fn-template17.C gcc/testsuite/g++.dg/cpp2a/fn-template17.C
new file mode 100644
index 00000000000..f0d24107682
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/fn-template17.C
@@ -0,0 +1,21 @@
+// PR c++/88184
+// { dg-do compile }
+// { dg-options "-std=c++2a -fchecking=2" }
+
+namespace A
+{
+  void f ();
+}
+
+using A::f;
+
+template <typename T> void g ()
+{
+  f<T> (); // { dg-error "no matching function for call" }
+}
+
+void
+fn ()
+{
+  g<int>();
+}
diff --git gcc/testsuite/g++.dg/cpp2a/fn-template18.C gcc/testsuite/g++.dg/cpp2a/fn-template18.C
new file mode 100644
index 00000000000..7fe5e89ace3
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/fn-template18.C
@@ -0,0 +1,23 @@
+// PR c++/88184
+// { dg-do compile }
+// { dg-options "-std=c++2a -fchecking=2" }
+
+namespace A
+{
+  void f ();
+  void f (int);
+  void f (int, int);
+}
+
+using A::f;
+
+template <typename T> void g ()
+{
+  f<T> (); // { dg-error "no matching function for call" }
+}
+
+void
+fn ()
+{
+  g<int>();
+}