[gcc r14-9759] c++: make __is_array return false for T[0] [PR114479]

[Marek Polacek via Gcc-cvs]

https://gcc.gnu.org/g:2f2924078ce51c2a0da3ad8f958f2d1de533969a

commit r14-9759-g2f2924078ce51c2a0da3ad8f958f2d1de533969a
Author: Marek Polacek <pola...@redhat.com>
Date:   Mon Apr 1 12:55:46 2024 -0400

    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.

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

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 9838331d2a9..0015ff4fb62 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 f1a6e08b87a..84993266629 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);