Mark switch expression as used to avoid bogus warning

        PR c/88660
        * c-parser.c (c_parser_switch_statement): Make sure to request
        marking the switch expr as used.

        PR c/88660
        * gcc.dg/pr88660.c: New test.

gcc/c/ChangeLog

+2020-01-30  Jeff Law  <law@redhat.com>
+
+	PR c/88660
+	* c-parser.c (c_parser_switch_statement): Make sure to request
+	marking the switch expr as used.
+
 2020-01-22  Joseph Myers  <joseph@codesourcery.com>
 
 	PR c/93348

gcc/c/c-parser.c

@@ -6607,7 +6607,7 @@ c_parser_switch_statement (c_parser *parser, bool *if_p)
 	  && c_token_starts_typename (c_parser_peek_2nd_token (parser)))
 	explicit_cast_p = true;
       ce = c_parser_expression (parser);
-      ce = convert_lvalue_to_rvalue (switch_cond_loc, ce, true, false);
+      ce = convert_lvalue_to_rvalue (switch_cond_loc, ce, true, true);
       expr = ce.value;
       /* ??? expr has no valid location?  */
       parens.skip_until_found_close (parser);

gcc/testsuite/ChangeLog

+2020-01-30  Jeff Law  <law@redhat.com
+
+	PR c/88660
+	* gcc.dg/pr88660.c: New test
+
 2020-01-30  Jakub Jelinek  <jakub@redhat.com>
 
 	PR lto/93384

gcc/testsuite/gcc.dg/pr88660.c

+/* { dg-do-compile } */
+/* { dg-options "-O -Wunused-but-set-variable" } */
+
+int main(void)
+{
+	const int i = 0;
+	switch(i)
+	{
+	default: break;
+	}
+}
+
+