c++: enable -Warray-compare by default

Discussion of P2865R5 mentioned that we only enabled this warning with
-Wall; we should handle it like other deprecations.

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Enable -Warray-compare
	in C++20.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/pr15791-1.C: Add -Wno-array-compare.

2 files changed, 6 insertions, 0 deletions

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index a002929..a02d011 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -1051,6 +1051,11 @@ c_common_post_options (const char **pfilename)
 		       warn_deprecated_literal_operator,
 		       deprecated_in (cxx23));
 
+  /* -Warray-compare is enabled by default in C++20.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       warn_array_compare,
+		       warn_array_compare || deprecated_in (cxx20));
+
   /* -Wtemplate-id-cdtor is enabled by default in C++20.  */
   SET_OPTION_IF_UNSET (&global_options, &global_options_set,
 		       warn_template_id_cdtor,
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr15791-1.C b/gcc/testsuite/g++.dg/tree-ssa/pr15791-1.C
index 68f14ad..9905851 100644
--- a/gcc/testsuite/g++.dg/tree-ssa/pr15791-1.C
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr15791-1.C
@@ -1,4 +1,5 @@
 /* { dg-do link } */
+/* { dg-additional-options -Wno-array-compare } */
 
 void link_error ();