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

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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.
This commit is contained in:
Marek Polacek 2024-04-01 12:55:46 -04:00
parent daa2e7c7ff
commit 2f2924078c
2 changed files with 19 additions and 1 deletions
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8
gcc/cp/semantics.cc
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@ -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);

12
gcc/testsuite/g++.dg/ext/is_array.C
View file

@ -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);