c++: address deduction and concepts [CWG2918]

CWG2918 changes deduction from an overload set for the case where multiple
candidates succeed and have the same type; previously this made the overload
set a non-deduced context, now it succeeds since the result is consistent
between the candidates.

This is needed for cases of overloading based on requirements, where we want
to choose the most constrained overload.  I also needed to adjust
resolve_address_of_overloaded_function accordingly; we already handled the
comparison for template candidates in most_specialized_instantiation, but
need to also do the comparison for non-template candidates such as member
functions of a class template.

	CWG 2918 (proposed)

gcc/cp/ChangeLog:

	* cp-tree.h (most_constrained_function): Declare..
	* class.cc (resolve_address_of_overloaded_function): Call it.
	* pt.cc (get_template_for_ordering): Handle list from
	resolve_address_of_overloaded_function.
	(most_constrained_function): No longer static.
	(resolve_overloaded_unification): Always compare type rather
	than decl.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2918.C: New test.

gcc/cp/class.cc

@@ -8896,11 +8896,18 @@ resolve_address_of_overloaded_function (tree target_type,
 	  tree match = most_specialized_instantiation (matches);
 
 	  if (match != error_mark_node)
-	    matches = tree_cons (TREE_PURPOSE (match),
-				 NULL_TREE,
-				 NULL_TREE);
+	    {
+	      matches = match;
+	      TREE_CHAIN (match) = NULL_TREE;
+	    }
 	}
     }
+  else if (flag_concepts && TREE_CHAIN (matches))
+    if (tree match = most_constrained_function (matches))
+      {
+	matches = match;
+	TREE_CHAIN (match) = NULL_TREE;
+      }
 
   /* Now we should have exactly one function in MATCHES.  */
   if (matches == NULL_TREE)

gcc/cp/cp-tree.h

@@ -7593,6 +7593,7 @@ extern int template_args_equal                  (tree, tree);
 extern tree maybe_process_partial_specialization (tree);
 extern tree most_specialized_instantiation	(tree);
 extern tree most_specialized_partial_spec       (tree, tsubst_flags_t, bool = false);
+extern tree most_constrained_function		(tree);
 extern void print_candidates			(tree);
 extern void instantiate_pending_templates	(int);
 extern tree tsubst_default_argument		(tree, int, tree, tree,

gcc/cp/pt.cc

@@ -2071,6 +2071,10 @@ get_template_for_ordering (tree list)
 {
   gcc_assert (TREE_CODE (list) == TREE_LIST);
   tree f = TREE_VALUE (list);
+  if (f == NULL_TREE)
+    /* Also handle a list from resolve_address_of_overloaded_function with the
+       function in TREE_PURPOSE.  */
+    f = TREE_PURPOSE (list);
   if (tree ti = DECL_TEMPLATE_INFO (f))
     return TI_TEMPLATE (ti);
   return f;
@@ -2084,7 +2088,7 @@ get_template_for_ordering (tree list)
 
    Note that we don't compare constraints on the functions
    themselves, but rather those of their templates. */
-static tree
+tree
 most_constrained_function (tree candidates)
 {
   // Try to find the best candidate in a first pass.
@@ -23865,11 +23869,14 @@ resolve_overloaded_unification (tree tparms,
 	tree fn = *iter;
 	if (flag_noexcept_type)
 	  maybe_instantiate_noexcept (fn, tf_none);
-	if (try_one_overload (tparms, targs, tempargs, parm, TREE_TYPE (fn),
+	if (TREE_CODE (fn) == FUNCTION_DECL && !constraints_satisfied_p (fn))
+	  continue;
+	tree elem = TREE_TYPE (fn);
+	if (try_one_overload (tparms, targs, tempargs, parm, elem,
 			      strict, sub_strict, addr_p, explain_p)
-	    && (!goodfn || !decls_match (goodfn, fn)))
+	    && (!goodfn || !same_type_p (goodfn, elem)))
 	  {
-	    goodfn = fn;
+	    goodfn = elem;
 	    ++good;
 	  }
       }
@@ -23879,6 +23886,9 @@ resolve_overloaded_unification (tree tparms,
      overloaded functions does not contain function templates and at most
      one of a set of overloaded functions provides a unique match.
 
+     CWG2918 allows multiple functions to match if they all have the same type,
+     so that we can choose the most constrained later.
+
      So if we found multiple possibilities, we return success but don't
      deduce anything.  */
 

gcc/testsuite/g++.dg/DRs/dr2918.C

@@ -0,0 +1,11 @@
+// CWG 2918 makes this OK
+// { dg-do compile { target c++20 } }
+
+template<bool B> struct X {
+  static void f(short) requires B; // #1
+  static void f(short);		   // #2
+};
+void test() {
+  auto x = &X<true>::f;		// OK, deduces void(*)(short), selects #1
+  auto y = &X<false>::f;	// OK, deduces void(*)(short), selects #2
+}