From 2f2924078ce51c2a0da3ad8f958f2d1de533969a Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Mon, 1 Apr 2024 12:55:46 -0400
Subject: c++: make __is_array return false for T[0] [PR114479]

When we switched to using the __is_array built-in trait to implement
std::is_array in r14-6623-g7fd9c349e45534, we started saying that
T[0] is an array.  There are various opinions as to whether that is
the best answer, but it seems prudent to keep the GCC 13 result.

	PR c++/114479

gcc/cp/ChangeLog:

	* semantics.cc (trait_expr_value) <case CPTK_IS_ARRAY>: Return false
	for zero-sized arrays.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_array.C: Extend.
---
 gcc/cp/semantics.cc                 |  8 +++++++-
 gcc/testsuite/g++.dg/ext/is_array.C | 12 ++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

(limited to 'gcc')

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 9838331..0015ff4 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12439,7 +12439,13 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
       return CP_AGGREGATE_TYPE_P (type1);
 
     case CPTK_IS_ARRAY:
-      return type_code1 == ARRAY_TYPE;
+      return (type_code1 == ARRAY_TYPE
+	      /* We don't want to report T[0] as being an array type.
+		 This is for compatibility with an implementation of
+		 std::is_array by template argument deduction, because
+		 compute_array_index_type_loc rejects a zero-size array
+		 in SFINAE context.  */
+	      && !(TYPE_SIZE (type1) && integer_zerop (TYPE_SIZE (type1))));
 
     case CPTK_IS_ASSIGNABLE:
       return is_xible (MODIFY_EXPR, type1, type2);
diff --git a/gcc/testsuite/g++.dg/ext/is_array.C b/gcc/testsuite/g++.dg/ext/is_array.C
index f1a6e08..8499326 100644
--- a/gcc/testsuite/g++.dg/ext/is_array.C
+++ b/gcc/testsuite/g++.dg/ext/is_array.C
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++11 } }
+// { dg-options "" }
 
 #define SA(X) static_assert((X),#X)
 
@@ -10,18 +11,29 @@
 
 class ClassType { };
 
+constexpr int sz0 = 0;
+constexpr int sz2 = 2;
+
 SA_TEST_CATEGORY(__is_array, int[2], true);
 SA_TEST_CATEGORY(__is_array, int[], true);
+SA_TEST_CATEGORY(__is_array, int[0], false);
 SA_TEST_CATEGORY(__is_array, int[2][3], true);
 SA_TEST_CATEGORY(__is_array, int[][3], true);
+SA_TEST_CATEGORY(__is_array, int[0][3], false);
+SA_TEST_CATEGORY(__is_array, int[3][0], false);
 SA_TEST_CATEGORY(__is_array, float*[2], true);
 SA_TEST_CATEGORY(__is_array, float*[], true);
 SA_TEST_CATEGORY(__is_array, float*[2][3], true);
 SA_TEST_CATEGORY(__is_array, float*[][3], true);
 SA_TEST_CATEGORY(__is_array, ClassType[2], true);
 SA_TEST_CATEGORY(__is_array, ClassType[], true);
+SA_TEST_CATEGORY(__is_array, ClassType[0], false);
 SA_TEST_CATEGORY(__is_array, ClassType[2][3], true);
 SA_TEST_CATEGORY(__is_array, ClassType[][3], true);
+SA_TEST_CATEGORY(__is_array, ClassType[0][3], false);
+SA_TEST_CATEGORY(__is_array, ClassType[2][0], false);
+SA_TEST_CATEGORY(__is_array, int[sz2], true);
+SA_TEST_CATEGORY(__is_array, int[sz0], false);
 
 // Sanity check.
 SA_TEST_CATEGORY(__is_array, ClassType, false);
-- 
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