c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

The following testcase shows various problems with -Wunused-but-set*
warnings and _Generic construct.  I think it is best to treat the selector
and the ignored expressions as (potentially) read, because when they are
parsed, the vars in there are already marked as TREE_USED.

2020-08-18  Jakub Jelinek  <jakub@redhat.com>

	PR c/96571
	* c-parser.c (c_parser_generic_selection): Change match_found from bool
	to int, holding index of the match.  Call mark_exp_read on the selector
	expression and on expressions other than the selected one.

	* gcc.dg/Wunused-var-4.c: New test.

2 files changed, 46 insertions, 6 deletions

diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 7961cbc98bb..a8bc301ffad 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -8686,7 +8686,7 @@ c_parser_generic_selection (c_parser *parser)
   struct c_expr selector, error_expr;
   tree selector_type;
   struct c_generic_association matched_assoc;
-  bool match_found = false;
+  int match_found = -1;
   location_t generic_loc, selector_loc;
 
   error_expr.original_code = ERROR_MARK;
@@ -8721,6 +8721,7 @@ c_parser_generic_selection (c_parser *parser)
       c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, NULL);
       return selector;
     }
+  mark_exp_read (selector.value);
   selector_type = TREE_TYPE (selector.value);
   /* In ISO C terms, rvalues (including the controlling expression of
      _Generic) do not have qualified types.  */
@@ -8820,18 +8821,18 @@ c_parser_generic_selection (c_parser *parser)
 
       if (assoc.type == NULL_TREE)
 	{
-	  if (!match_found)
+	  if (match_found < 0)
 	    {
 	      matched_assoc = assoc;
-	      match_found = true;
+	      match_found = associations.length ();
 	    }
 	}
       else if (comptypes (assoc.type, selector_type))
 	{
-	  if (!match_found || matched_assoc.type == NULL_TREE)
+	  if (match_found < 0 || matched_assoc.type == NULL_TREE)
 	    {
 	      matched_assoc = assoc;
-	      match_found = true;
+	      match_found = associations.length ();
 	    }
 	  else
 	    {
@@ -8849,13 +8850,19 @@ c_parser_generic_selection (c_parser *parser)
       c_parser_consume_token (parser);
     }
 
+  unsigned int ix;
+  struct c_generic_association *iter;
+  FOR_EACH_VEC_ELT (associations, ix, iter)
+    if (ix != (unsigned) match_found)
+      mark_exp_read (iter->expression.value);
+
   if (!parens.require_close (parser))
     {
       c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, NULL);
       return error_expr;
     }
 
-  if (!match_found)
+  if (match_found < 0)
     {
       error_at (selector_loc, "%<_Generic%> selector of type %qT is not "
 		"compatible with any association",
diff --git a/gcc/testsuite/gcc.dg/Wunused-var-4.c b/gcc/testsuite/gcc.dg/Wunused-var-4.c
new file mode 100644
index 00000000000..08ddcf4407e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wunused-var-4.c
@@ -0,0 +1,33 @@
+/* PR c/96571 */
+/* { dg-do compile } */
+/* { dg-options "-std=c99 -O2 -Wunused-but-set-variable" } */
+
+enum E { V };
+
+int
+foo (void)
+{
+  enum E v;				/* { dg-bogus "set but not used" } */
+  return _Generic (v, enum E : 0);
+}
+
+int
+bar (void)
+{
+  int a = 0;				/* { dg-bogus "set but not used" } */
+  return _Generic (0, int : a);
+}
+
+int
+baz (void)
+{
+  int a;				/* { dg-bogus "set but not used" } */
+  return _Generic (0, long long : a, int : 0);
+}
+
+int
+qux (void)
+{
+  int a;				/* { dg-bogus "set but not used" } */
+  return _Generic (0, long long : a, default: 0);
+}