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

[Marek Polacek <polacek at redhat dot com>]

On Thu, Nov 29, 2018 at 04:03:50PM -0500, Jason Merrill wrote:
> On 11/29/18 1:28 PM, Marek Polacek wrote:
> > 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.
> 
> Would it work for lookup_template_function to always use unknown_type_node?

Luckily, that seems to work!  I wish I'd thought of that, I like fixing bugs by
simplifying code.

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

2018-12-04  Marek Polacek  <polacek@redhat.com>

	PR c++/88184 - ICE when treating name as template-name.
	* pt.c (lookup_template_function): Always build the TEMPLATE_ID_EXPR
	with unknown_type_node.

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

diff --git gcc/cp/pt.c gcc/cp/pt.c
index a0d899f594b..78addc6c1c2 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -9068,8 +9068,6 @@ add_pending_template (tree d)
 tree
 lookup_template_function (tree fns, tree arglist)
 {
-  tree type;
-
   if (fns == error_mark_node || arglist == error_mark_node)
     return error_mark_node;
 
@@ -9090,11 +9088,7 @@ lookup_template_function (tree fns, tree arglist)
       return fns;
     }
 
-  type = TREE_TYPE (fns);
-  if (TREE_CODE (fns) == OVERLOAD || !type)
-    type = unknown_type_node;
-
-  return build2 (TEMPLATE_ID_EXPR, type, fns, arglist);
+  return build2 (TEMPLATE_ID_EXPR, unknown_type_node, fns, arglist);
 }
 
 /* Within the scope of a template class S<T>, the name S gets bound
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>();
+}