c-family: Implement -Warray-compare [PR97573]

This patch addresses one of my leftovers from GCC 11.  C++20 introduced
[depr.array.comp]: "Equality and relational comparisons between two operands
of array type are deprecated." so this patch adds -Warray-compare.  Since the
code in question is dubious (the comparison doesn't actually compare the array
elements), I've added this warning for C too, and enabled it in all C++ modes.

	PR c++/97573

gcc/c-family/ChangeLog:

	* c-common.h (do_warn_array_compare): Declare.
	* c-warn.c (do_warn_array_compare): New.
	* c.opt (Warray-compare): New option.

gcc/c/ChangeLog:

	* c-typeck.c (parser_build_binary_op): Call do_warn_array_compare.

gcc/cp/ChangeLog:

	* typeck.c (cp_build_binary_op): Call do_warn_array_compare.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Warray-compare.

gcc/testsuite/ChangeLog:

	* c-c++-common/Warray-compare-1.c: New test.
	* c-c++-common/Warray-compare-2.c: New test.

1 files changed, 32 insertions, 0 deletions

diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index 84ad663..99cde4a 100644
--- a/gcc/c-family/c-warn.c
+++ b/gcc/c-family/c-warn.c
@@ -3726,3 +3726,35 @@ maybe_warn_sizeof_array_div (location_t loc, tree arr, tree arr_type,
 	}
     }
 }
+
+/* Warn about C++20 [depr.array.comp] array comparisons: "Equality
+   and relational comparisons between two operands of array type are
+   deprecated."  We also warn in C and earlier C++ standards.  CODE is
+   the code for this comparison, OP0 and OP1 are the operands.  */
+
+void
+do_warn_array_compare (location_t location, tree_code code, tree op0, tree op1)
+{
+  STRIP_NOPS (op0);
+  STRIP_NOPS (op1);
+  if (TREE_CODE (op0) == ADDR_EXPR)
+    op0 = TREE_OPERAND (op0, 0);
+  if (TREE_CODE (op1) == ADDR_EXPR)
+    op1 = TREE_OPERAND (op1, 0);
+
+  auto_diagnostic_group d;
+  if (warning_at (location, OPT_Warray_compare,
+		  (c_dialect_cxx () && cxx_dialect >= cxx20)
+		  ? G_("comparison between two arrays is deprecated in C++20")
+		  : G_("comparison between two arrays")))
+    {
+      /* C doesn't allow +arr.  */
+      if (c_dialect_cxx ())
+	inform (location, "use unary %<+%> which decays operands to pointers "
+		"or %<&%D[0] %s &%D[0]%> to compare the addresses",
+		op0, op_symbol_code (code), op1);
+      else
+	inform (location, "use %<&%D[0] %s &%D[0]%> to compare the addresses",
+		op0, op_symbol_code (code), op1);
+    }
+}