c++: remove redundant warning [PR100879]

Before my r277864, build_new_op promoted enums to int before passing them on
to cp_build_binary_op; after that commit, it doesn't, so
warn_for_sign_compare sees the enum operands and gives a redundant warning.
This warning dates back to 1995, and seems to have been dead code for a long
time--likely since build_new_op was added in 1997--so let's just remove it.

	PR c++/100879

gcc/c-family/ChangeLog:

	* c-warn.c (warn_for_sign_compare): Remove C++ enum mismatch
	warning.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/enum3.C: New test.

2 files changed, 9 insertions, 12 deletions

diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index a587b99..cd3c99e 100644
--- a/gcc/c-family/c-warn.c
+++ b/gcc/c-family/c-warn.c
@@ -2240,18 +2240,6 @@ warn_for_sign_compare (location_t location,
   int op1_signed = !TYPE_UNSIGNED (TREE_TYPE (orig_op1));
   int unsignedp0, unsignedp1;
 
-  /* In C++, check for comparison of different enum types.  */
-  if (c_dialect_cxx()
-      && TREE_CODE (TREE_TYPE (orig_op0)) == ENUMERAL_TYPE
-      && TREE_CODE (TREE_TYPE (orig_op1)) == ENUMERAL_TYPE
-      && TYPE_MAIN_VARIANT (TREE_TYPE (orig_op0))
-	 != TYPE_MAIN_VARIANT (TREE_TYPE (orig_op1)))
-    {
-      warning_at (location,
-		  OPT_Wsign_compare, "comparison between types %qT and %qT",
-		  TREE_TYPE (orig_op0), TREE_TYPE (orig_op1));
-    }
-
   /* Do not warn if the comparison is being done in a signed type,
      since the signed type will only be chosen if it can represent
      all the values of the unsigned type.  */
diff --git a/gcc/testsuite/g++.dg/diagnostic/enum3.C b/gcc/testsuite/g++.dg/diagnostic/enum3.C
new file mode 100644
index 0000000..d51aa8a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/enum3.C
@@ -0,0 +1,9 @@
+// PR c++/100879
+// { dg-additional-options -Werror=sign-compare }
+
+enum e1 { e1val };
+enum e2 { e3val };
+
+int main( int, char * [] ) {
+   if ( e1val == e3val ) return 1; // { dg-warning -Wenum-compare }
+}