c++: Fix missing SFINAE when binding a bit-field to a reference (PR 93729)

We are unconditionally emitting an error here, without first checking complain.

gcc/cp/ChangeLog:

	PR c++/93729
	* call.c (convert_like_real): Check complain before emitting an error
	about binding a bit-field to a reference.

gcc/testsuite/ChangeLog:

	PR c++/93729
	* g++.dg/concepts/pr93729.C: New test.

gcc/cp/ChangeLog

 2020-03-08  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/93729
+	* call.c (convert_like_real): Check complain before emitting an error
+	about binding a bit-field to a reference.
+
 	* cxx-pretty-print.c (cxx_pretty_printer::simple_type_specifier)
 	[TYPENAME_TYPE]: Print the TYPENAME_TYPE_FULLNAME instead of the
 	TYPE_NAME.

gcc/cp/call.c

@@ -7730,6 +7730,8 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
 	      {
 		/* If the reference is volatile or non-const, we
 		   cannot create a temporary.  */
+		if (complain & tf_error)
+		  {
 		    if (lvalue & clk_bitfield)
 		      error_at (loc, "cannot bind bit-field %qE to %qT",
 				expr, ref_type);
@@ -7739,6 +7741,7 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
 		    else
 		      error_at (loc, "cannot bind rvalue %qE to %qT",
 				expr, ref_type);
+		  }
 		return error_mark_node;
 	      }
 	    /* If the source is a packed field, and we must use a copy

gcc/testsuite/ChangeLog

 2020-03-08  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/93729
+	* g++.dg/concepts/pr93729.C: New test.
+
 	* g++.dg/concepts/diagnostic4.C: New test.
 
 2020-03-08  H.J. Lu  <hongjiu.lu@intel.com>

gcc/testsuite/g++.dg/concepts/pr93729.C

+// { dg-do compile { target c++2a } }
+
+// PR c++/93729
+
+struct B
+{
+  int a:4;
+  int b:4;
+};
+
+template<typename T>
+concept c1
+  = requires(T x, void(f)(int &)) { f(x.a); }; // { dg-bogus "cannot bind" }
+
+static_assert(!c1<B>);