c++: CTAD and deduction guide selection [PR86439]

During CTAD, we select the best viable deduction guide using
build_new_function_call, which performs overload resolution on the set
of candidate guides and then forms a call to the guide.  As the PR
points out, this latter step is unnecessary and occasionally incorrect
since a call to the selected guide may be ill-formed, or forming the
call may have side effects such as prematurely deducing the type of a {}.

So this patch introduces a specialized subroutine based on
build_new_function_call that stops short of building a call to the
selected function, and makes do_class_deduction use this subroutine
instead.  And since a call is no longer built, do_class_deduction
doesn't need to set tf_decltype or cp_unevaluated_operand anymore.

This change causes us to reject some container CTAD examples in the
libstdc++ testsuite due to deduction failure for {}, which AFAICT is the
correct behavior.  Previously in e.g. the first removed example

  std::map{{std::pair{1, 2.0}, {2, 3.0}, {3, 4.0}}, {}},

the type of the {} would get deduced to less<int> as a side effect of
forming a call to the chosen guide

  template<typename _Key, typename _Tp, typename _Compare = less<_Key>,
           typename _Allocator = allocator<pair<const _Key, _Tp>>>
      map(initializer_list<pair<_Key, _Tp>>,
          _Compare = _Compare(), _Allocator = _Allocator())
      -> map<_Key, _Tp, _Compare, _Allocator>;

which made later overload resolution for the constructor call
unambiguous.  Now, the type of the {} remains undeduced until
constructor overload resolution, and we complain about ambiguity
for the two equally good constructor candidates

  map(initializer_list<value_type>,
      const _Compare& = _Compare(),
      const allocator_type& = allocator_type())

  map(initializer_list<value_type>, const allocator_type&).

This patch fixes these problematic container CTAD examples by giving
the {} an appropriate concrete type.  Two of these adjusted CTAD
examples (one for std::set and one for std::multiset) end up triggering
an unrelated CTAD bug on trunk, PR101174, so these two adjusted examples
are commented out for now.

	PR c++/86439

gcc/cp/ChangeLog:

	* call.c (print_error_for_call_failure): Constify 'args' parameter.
	(perform_dguide_overload_resolution): Define.
	* cp-tree.h: (perform_dguide_overload_resolution): Declare.
	* pt.c (do_class_deduction): Use perform_dguide_overload_resolution
	instead of build_new_function_call.  Don't use tf_decltype or
	set cp_unevaluated_operand.  Remove unnecessary NULL_TREE tests.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/map/cons/deduction.cc: Replace ambiguous
	CTAD examples.
	* testsuite/23_containers/multimap/cons/deduction.cc: Likewise.
	* testsuite/23_containers/multiset/cons/deduction.cc: Likewise.
	Mention one of the replaced examples is broken due to PR101174.
	* testsuite/23_containers/set/cons/deduction.cc: Likewise.
	* testsuite/23_containers/unordered_map/cons/deduction.cc: Replace
	ambiguous CTAD examples.
	* testsuite/23_containers/unordered_multimap/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/cons/deduction.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction88.C: New test.
	* g++.dg/cpp1z/class-deduction89.C: New test.
	* g++.dg/cpp1z/class-deduction90.C: New test.

6 files changed, 100 insertions, 28 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 9f03534..aafc7ac 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -4629,7 +4629,7 @@ perform_overload_resolution (tree fn,
    functions.  */
 
 static void
-print_error_for_call_failure (tree fn, vec<tree, va_gc> *args,
+print_error_for_call_failure (tree fn, const vec<tree, va_gc> *args,
 			      struct z_candidate *candidates)
 {
   tree targs = NULL_TREE;
@@ -4654,6 +4654,40 @@ print_error_for_call_failure (tree fn, vec<tree, va_gc> *args,
     print_z_candidates (loc, candidates);
 }
 
+/* Perform overload resolution on the set of deduction guides DGUIDES
+   using ARGS.  Returns the selected deduction guide, or error_mark_node
+   if overload resolution fails.  */
+
+tree
+perform_dguide_overload_resolution (tree dguides, const vec<tree, va_gc> *args,
+				    tsubst_flags_t complain)
+{
+  z_candidate *candidates;
+  bool any_viable_p;
+  tree result;
+
+  gcc_assert (deduction_guide_p (OVL_FIRST (dguides)));
+
+  /* Get the high-water mark for the CONVERSION_OBSTACK.  */
+  void *p = conversion_obstack_alloc (0);
+
+  z_candidate *cand = perform_overload_resolution (dguides, args, &candidates,
+						   &any_viable_p, complain);
+  if (!cand)
+    {
+      if (complain & tf_error)
+	print_error_for_call_failure (dguides, args, candidates);
+      result = error_mark_node;
+    }
+  else
+    result = cand->fn;
+
+  /* Free all the conversions we allocated.  */
+  obstack_free (&conversion_obstack, p);
+
+  return result;
+}
+
 /* Return an expression for a call to FN (a namespace-scope function,
    or a static member function) with the ARGS.  This may change
    ARGS.  */
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 36f99cc..6f71371 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6437,6 +6437,8 @@ extern void complain_about_bad_argument	(location_t arg_loc,
 						 tree from_type, tree to_type,
 						 tree fndecl, int parmnum);
 extern void maybe_inform_about_fndecl_for_bogus_argument_init (tree, int);
+extern tree perform_dguide_overload_resolution	(tree, const vec<tree, va_gc> *,
+						 tsubst_flags_t);
 
 
 /* A class for recording information about access failures (e.g. private
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 15947b2..732fb40 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29382,7 +29382,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
     if (tree guide = maybe_aggr_guide (tmpl, init, args))
       cands = lookup_add (guide, cands);
 
-  tree call = error_mark_node;
+  tree fndecl = error_mark_node;
 
   /* If this is list-initialization and the class has a list constructor, first
      try deducing from the list as a single argument, as [over.match.list].  */
@@ -29396,11 +29396,9 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
       }
   if (list_cands)
     {
-      ++cp_unevaluated_operand;
-      call = build_new_function_call (list_cands, &args, tf_decltype);
-      --cp_unevaluated_operand;
+      fndecl = perform_dguide_overload_resolution (list_cands, args, tf_none);
 
-      if (call == error_mark_node)
+      if (fndecl == error_mark_node)
 	{
 	  /* That didn't work, now try treating the list as a sequence of
 	     arguments.  */
@@ -29416,31 +29414,22 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 	     "user-declared constructors", type);
       return error_mark_node;
     }
-  else if (!cands && call == error_mark_node)
+  else if (!cands && fndecl == error_mark_node)
     {
       error ("cannot deduce template arguments of %qT, as it has no viable "
 	     "deduction guides", type);
       return error_mark_node;
     }
 
-  if (call == error_mark_node)
-    {
-      ++cp_unevaluated_operand;
-      call = build_new_function_call (cands, &args, tf_decltype);
-      --cp_unevaluated_operand;
-    }
+  if (fndecl == error_mark_node)
+    fndecl = perform_dguide_overload_resolution (cands, args, tf_none);
 
-  if (call == error_mark_node)
+  if (fndecl == error_mark_node)
     {
       if (complain & tf_warning_or_error)
 	{
 	  error ("class template argument deduction failed:");
-
-	  ++cp_unevaluated_operand;
-	  call = build_new_function_call (cands, &args,
-					  complain | tf_decltype);
-	  --cp_unevaluated_operand;
-
+	  perform_dguide_overload_resolution (cands, args, complain);
 	  if (elided)
 	    inform (input_location, "explicit deduction guides not considered "
 		    "for copy-initialization");
@@ -29451,8 +29440,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
      constructor is chosen, the initialization is ill-formed.  */
   else if (flags & LOOKUP_ONLYCONVERTING)
     {
-      tree fndecl = cp_get_callee_fndecl_nofold (call);
-      if (fndecl && DECL_NONCONVERTING_P (fndecl))
+      if (DECL_NONCONVERTING_P (fndecl))
 	{
 	  if (complain & tf_warning_or_error)
 	    {
@@ -29470,12 +29458,10 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 
   /* If CTAD succeeded but the type doesn't have any explicit deduction
      guides, this deduction might not be what the user intended.  */
-  if (call != error_mark_node && !any_dguides_p)
+  if (fndecl != error_mark_node && !any_dguides_p)
     {
-      tree fndecl = cp_get_callee_fndecl_nofold (call);
-      if (fndecl != NULL_TREE
-	  && (!DECL_IN_SYSTEM_HEADER (fndecl)
-	      || global_dc->dc_warn_system_headers)
+      if ((!DECL_IN_SYSTEM_HEADER (fndecl)
+	   || global_dc->dc_warn_system_headers)
 	  && warning (OPT_Wctad_maybe_unsupported,
 		      "%qT may not intend to support class template argument "
 		      "deduction", type))
@@ -29483,7 +29469,8 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 		"warning");
     }
 
-  return cp_build_qualified_type (TREE_TYPE (call), cp_type_quals (ptype));
+  return cp_build_qualified_type (TREE_TYPE (TREE_TYPE (fndecl)),
+				  cp_type_quals (ptype));
 }
 
 /* Replace occurrences of 'auto' in TYPE with the appropriate type deduced
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction88.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction88.C
new file mode 100644
index 0000000..be38fed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction88.C
@@ -0,0 +1,18 @@
+// PR c++/86439
+// { dg-do compile { target c++17 } }
+
+struct NC {
+  NC() = default;
+  NC(NC const&) = delete;
+  NC& operator=(NC const&) = delete;
+};
+
+template <int>
+struct C {
+  C(NC const&);
+};
+
+C(NC) -> C<0>;
+
+NC nc;
+C c(nc);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction89.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction89.C
new file mode 100644
index 0000000..dd89857
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction89.C
@@ -0,0 +1,15 @@
+// PR c++/86439
+// { dg-do compile { target c++17 } }
+
+struct B { };
+struct C { };
+
+template<class T>
+struct A {
+  A(T, B);
+};
+
+template<class T>
+A(T, C) -> A<T>;
+
+A a(0, {});
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction90.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction90.C
new file mode 100644
index 0000000..8b93193
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction90.C
@@ -0,0 +1,16 @@
+// PR c++/86439
+// { dg-do compile { target c++17 } }
+
+struct less { };
+struct allocator { };
+
+template<class T, class U = less, class V = allocator>
+struct A {
+  A(T, U);
+  A(T, V);
+};
+
+template<class T, class U = less>
+A(T, U) -> A<T>;
+
+A a(0, {}); // { dg-error "ambiguous" }