c++: CTAD and explicit deduction guides for copy-list-init [PR90210]

This PR points out that we accept

  template<typename T> struct tuple { tuple(T); }; // #1
  template<typename T> explicit tuple(T t) -> tuple<T>; // #2
  tuple t = { 1 };

despite the 'explicit' deduction guide in a copy-list-initialization
context.  That's because in deduction_guides_for we first find the
user-defined deduction guide (#2), and then ctor_deduction_guides_for
creates artificial deduction guides: one from the tuple(T) constructor and
a copy guide.  So we end up with these three guides:

  (1) template<class T> tuple(T) -> tuple<T> [DECL_NONCONVERTING_P]
  (2) template<class T> tuple(tuple<T>) -> tuple<T>
  (3) template<class T> tuple(T) -> tuple<T>

Then, in do_class_deduction, we prune this set, and get rid of (1).
Then overload resolution selects (3) and we succeed.

But [over.match.list]p1 says "In copy-list-initialization, if an explicit
constructor is chosen, the initialization is ill-formed."  It also goes
on to say that this differs from other situations where only converting
constructors are considered for copy-initialization.  Therefore for
list-initialization we consider explicit constructors and complain if one
is chosen.  E.g. convert_like_internal/ck_user can give an error.

So my logic runs that we should not prune the deduction_guides_for guides
in a copy-list-initialization context, and only complain if we actually
choose an explicit deduction guide.  This matches clang++/EDG/msvc++.

gcc/cp/ChangeLog:

	PR c++/90210
	* pt.c (do_class_deduction): Don't prune explicit deduction guides
	in copy-list-initialization.  In copy-list-initialization, if an
	explicit deduction guide was selected, give an error.

gcc/testsuite/ChangeLog:

	PR c++/90210
	* g++.dg/cpp1z/class-deduction73.C: New test.

gcc/cp/pt.c

@@ -28977,6 +28977,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   tree type = TREE_TYPE (tmpl);
 
   bool try_list_ctor = false;
+  bool list_init_p = false;
 
   releasing_vec rv_args = NULL;
   vec<tree,va_gc> *&args = *&rv_args;
@@ -28984,6 +28985,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
     args = make_tree_vector ();
   else if (BRACE_ENCLOSED_INITIALIZER_P (init))
     {
+      list_init_p = true;
       try_list_ctor = TYPE_HAS_LIST_CTOR (type);
       if (try_list_ctor && CONSTRUCTOR_NELTS (init) == 1)
 	{
@@ -29016,9 +29018,10 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   if (cands == error_mark_node)
     return error_mark_node;
 
-  /* Prune explicit deduction guides in copy-initialization context.  */
+  /* Prune explicit deduction guides in copy-initialization context (but
+     not copy-list-initialization).  */
   bool elided = false;
-  if (flags & LOOKUP_ONLYCONVERTING)
+  if (!list_init_p && (flags & LOOKUP_ONLYCONVERTING))
     {
       for (lkp_iterator iter (cands); !elided && iter; ++iter)
 	if (DECL_NONCONVERTING_P (STRIP_TEMPLATE (*iter)))
@@ -29087,18 +29090,42 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
       --cp_unevaluated_operand;
     }
 
-  if (call == error_mark_node
-      && (complain & tf_warning_or_error))
+  if (call == error_mark_node)
     {
-      error ("class template argument deduction failed:");
+      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;
+	  ++cp_unevaluated_operand;
+	  call = build_new_function_call (cands, &args,
+					  complain | tf_decltype);
+	  --cp_unevaluated_operand;
 
-      if (elided)
-	inform (input_location, "explicit deduction guides not considered "
-		"for copy-initialization");
+	  if (elided)
+	    inform (input_location, "explicit deduction guides not considered "
+		    "for copy-initialization");
+	}
+      return error_mark_node;
+    }
+  /* [over.match.list]/1: In copy-list-initialization, if an explicit
+     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 (complain & tf_warning_or_error)
+	    {
+	      // TODO: Pass down location from cp_finish_decl.
+	      error ("class template argument deduction for %qT failed: "
+		     "explicit deduction guide selected in "
+		     "copy-list-initialization", type);
+	      inform (DECL_SOURCE_LOCATION (fndecl),
+		      "explicit deduction guide declared here");
+
+	    }
+	  return error_mark_node;
+	}
     }
 
   /* If CTAD succeeded but the type doesn't have any explicit deduction

gcc/testsuite/g++.dg/cpp1z/class-deduction73.C

@@ -0,0 +1,41 @@
+// PR c++/90210
+// { dg-do compile { target c++17 } }
+
+template<typename T> struct tuple { tuple(T); };
+template<typename T> explicit tuple(T t) -> tuple<T>;
+tuple t = { 1 }; // { dg-error "explicit deduction guide selected" }
+tuple t1 = tuple{ 1 };
+tuple t2{ 1 };
+
+template<typename T> struct A { A(T, T); };
+template<typename T> explicit A(T, T) -> A<int>;
+A a = {1, 1}; // { dg-error "explicit deduction guide selected" }
+A a1 = A{1, 1};
+A a2{1, 1};
+
+template<typename T, typename U>
+struct B {
+  B(T, U);
+};
+template<typename T, typename U>
+B(T, U) -> B<T, typename U::type>; // SFINAEd-out
+B b = { 1, 2 }; // OK
+B b1 = B{ 1, 2 }; // OK
+B b2{ 1, 2 }; // OK
+
+// Overriden implicit default constructor deduction guide:
+template<typename T>
+struct C { };
+explicit C() -> C<int>;
+C c = {}; // { dg-error "explicit deduction guide selected" }
+C c1 = C{};
+C c2{};
+
+// Overriden copy guide:
+template<typename T>
+struct D { };
+template<typename T> explicit D(D<T>) -> D<T>;
+D<int> d;
+D d1 = {d}; // { dg-error "explicit deduction guide selected" }
+D d2 = D{d};
+D d3{d};