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

[Jason Merrill <jason@redhat.com>]

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