From cde17323f950ac372691efd0a740fe0b4d7914a4 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 18 Jul 2023 09:22:49 -0400
Subject: [PATCH] c++: non-standalone surrogate call template

I noticed we're accidentally prevented from considering a pointer- or
reference-to-function conversion function template if it's not the first
conversion function that's considered, which for the testcase below
results in us accepting the B call but not the A call despite the only
difference between A and B being their order of member declarations.
This patch fixes this so that the outcome of overload resolution doesn't
arbitrarily depend on declaration order here.

gcc/cp/ChangeLog:

	* call.cc (add_template_conv_candidate): Don't check for
	non-empty 'candidates' here.
	(build_op_call): Check it here, before we've considered any
	conversion functions.

gcc/testsuite/ChangeLog:

	* g++.dg/overload/conv-op5.C: New test.
---
 gcc/cp/call.cc                           | 24 ++++++++++++++----------
 gcc/testsuite/g++.dg/overload/conv-op5.C | 17 +++++++++++++++++
 2 files changed, 31 insertions(+), 10 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/overload/conv-op5.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 24f93dd983c..b55230d98aa 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -3709,12 +3709,6 @@ add_template_conv_candidate (struct z_candidate **candidates, tree tmpl,
 			     tree return_type, tree access_path,
 			     tree conversion_path, tsubst_flags_t complain)
 {
-  /* Making this work broke PR 71117 and 85118, so until the committee resolves
-     core issue 2189, let's disable this candidate if there are any call
-     operators.  */
-  if (*candidates)
-    return NULL;
-
   return
     add_template_candidate_real (candidates, tmpl, NULL_TREE, NULL_TREE,
 				 NULL_TREE, arglist, return_type, access_path,
@@ -5290,6 +5284,8 @@ build_op_call (tree obj, vec<tree, va_gc> **args, tsubst_flags_t complain)
 		      LOOKUP_NORMAL, &candidates, complain);
     }
 
+  bool any_call_ops = candidates != nullptr;
+
   convs = lookup_conversions (type);
 
   for (; convs; convs = TREE_CHAIN (convs))
@@ -5306,10 +5302,18 @@ build_op_call (tree obj, vec<tree, va_gc> **args, tsubst_flags_t complain)
 	      continue;
 
 	    if (TREE_CODE (fn) == TEMPLATE_DECL)
-	      add_template_conv_candidate
-		(&candidates, fn, obj, *args, totype,
-		 /*access_path=*/NULL_TREE,
-		 /*conversion_path=*/NULL_TREE, complain);
+	      {
+		/* Making this work broke PR 71117 and 85118, so until the
+		   committee resolves core issue 2189, let's disable this
+		   candidate if there are any call operators.  */
+		if (any_call_ops)
+		  continue;
+
+		add_template_conv_candidate
+		  (&candidates, fn, obj, *args, totype,
+		   /*access_path=*/NULL_TREE,
+		   /*conversion_path=*/NULL_TREE, complain);
+	      }
 	    else
 	      add_conv_candidate (&candidates, fn, obj,
 				  *args, /*conversion_path=*/NULL_TREE,
diff --git a/gcc/testsuite/g++.dg/overload/conv-op5.C b/gcc/testsuite/g++.dg/overload/conv-op5.C
new file mode 100644
index 00000000000..4f7509257d6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/overload/conv-op5.C
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++11 } }
+
+template<class T> using F = int(*)(T);
+using G = int(*)(int*);
+
+struct A {
+  template<class T> operator F<T>(); // #1
+  operator G() = delete; // #2
+};
+
+struct B {
+  operator G() = delete; // #2
+  template<class T> operator F<T>(); // #1
+};
+
+int i = A{}(0); // OK, selects #1
+int j = B{}(0); // OK, selects #1