c++: Don't allow type-constraint auto(x) [PR104752]

104752 points out that

  template<class T>
  concept C = true;
  auto y = C auto(1);

is ill-formed as per [dcl.type.auto.deduct]: "For an explicit type conversion,
T is the specified type, which shall be auto." which doesn't allow
type-constraint auto.

	PR c++/104752

gcc/cp/ChangeLog:

	* semantics.cc (finish_compound_literal): Disallow auto{x} for
	is_constrained_auto.
	* typeck2.cc (build_functional_cast_1): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast12.C: New test.

gcc/cp/semantics.cc

@@ -3150,7 +3150,13 @@ finish_compound_literal (tree type, tree compound_literal,
 	   && !AUTO_IS_DECLTYPE (type)
 	   && CONSTRUCTOR_NELTS (compound_literal) == 1)
     {
-      if (cxx_dialect < cxx23)
+      if (is_constrained_auto (type))
+	{
+	  if (complain & tf_error)
+	    error ("%<auto{x}%> cannot be constrained");
+	  return error_mark_node;
+	}
+      else if (cxx_dialect < cxx23)
 	pedwarn (input_location, OPT_Wc__23_extensions,
 		 "%<auto{x}%> only available with "
 		 "%<-std=c++2b%> or %<-std=gnu++2b%>");

gcc/cp/typeck2.cc

@@ -2308,7 +2308,13 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 	       && list_length (parms) == 1)
 	{
 	  init = TREE_VALUE (parms);
-	  if (cxx_dialect < cxx23)
+	  if (is_constrained_auto (anode))
+	    {
+	      if (complain & tf_error)
+		error_at (loc, "%<auto(x)%> cannot be constrained");
+	      return error_mark_node;
+	    }
+	  else if (cxx_dialect < cxx23)
 	    pedwarn (loc, OPT_Wc__23_extensions,
 		     "%<auto(x)%> only available with "
 		     "%<-std=c++2b%> or %<-std=gnu++2b%>");

gcc/testsuite/g++.dg/cpp23/auto-fncast12.C

@@ -0,0 +1,8 @@
+// PR c++/104752
+// { dg-do compile { target c++23 } }
+
+template<class T>
+concept C = true;
+auto x = auto(1);     // valid (P0849R8)
+auto y = C auto(1);   // { dg-error "cannot be constrained" }
+auto z = C auto{1};   // { dg-error "cannot be constrained" }