c++: Fix diagnostic for binding lvalue reference to volatile rvalue [PR 100635]

The current diagnostic assumes the reference binding fails because the
reference is non-const, but it can also fail if the rvalue is volatile.

Use the current diagnostic for non-const cases, and a modified
diagnostic otherwise.

gcc/cp/ChangeLog:

	PR c++/100635
	* call.c (convert_like_internal): Print different diagnostic if
	the lvalue reference is const.

gcc/testsuite/ChangeLog:

	* g++.dg/conversion/pr100635.C: New test.

gcc/cp/call.c

@@ -7900,9 +7900,13 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
 			      "type %qH to a value of type %qI",
 			      totype, next->type);
 		  }
-		else
+		else if (!CP_TYPE_CONST_P (TREE_TYPE (ref_type)))
 		  error_at (loc, "cannot bind non-const lvalue reference of "
 			    "type %qH to an rvalue of type %qI", totype, extype);
+		else // extype is volatile
+		  error_at (loc, "cannot bind lvalue reference of type "
+			    "%qH to an rvalue of type %qI", totype,
+			    extype);
 	      }
 	    else if (!reference_compatible_p (TREE_TYPE (totype), extype))
 	      {

gcc/testsuite/g++.dg/conversion/pr100635.C

@@ -0,0 +1,12 @@
+// PR c++/100635
+// { dg-do compile }
+// { dg-additional-options "-Wno-volatile" { target c++2a } }
+
+struct S { };
+volatile S v();
+const volatile S& svol = v(); // { dg-error "cannot bind lvalue reference of type 'const volatile S&' to an rvalue of type 'volatile S'" }
+
+#if __cplusplus >= 201103L
+volatile int&& declvol();
+const volatile int& voli = declvol(); // { dg-error "cannot bind lvalue reference of type 'const volatile int&' to an rvalue of type 'volatile int'" "" { target c++11} }
+#endif