Re: [PATCH] c++: make __is_array return false for T[0] [PR114479]

[Jason Merrill]

On 4/1/24 13:50, Marek Polacek wrote:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
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                 |  4 +++-
  gcc/testsuite/g++.dg/ext/is_array.C | 12 ++++++++++++
  2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 9838331d2a9..f561c119dfd 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12439,7 +12439,9 @@ 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.  */

Please elaborate that this is for compatibility with an implementation 
of is_array by template argument deduction, because 
compute_array_index_type_loc rejects a zero-size array in SFINAE context.

OK with that adjustment.

+             && !(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);

base-commit: bba118db3f63cb1e3953a014aa3ac2ad89908950