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Core: Make structured-bindings tuple_size SFINAE-friendly for dynamic sizes

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libeigen/eigen!2828

Closes #3103

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>
This commit is contained in:
Rasmus Munk Larsen
2026-08-17 10:12:24 -07:00
co-authored by Rasmus Munk Larsen
parent 14d33f2f2e
commit 76a7c53018
4 changed files with 74 additions and 37 deletions
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test/structured_bindings.cpp
+31
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@@ -188,6 +188,36 @@ void check_tuple_element() {
STATIC_CHECK((std::is_same<std::tuple_element_t<0, Array3i>, int>::value));
}
// Emulates generic tuple-like detection as done by fmt's range formatter
// (issue #3103): probing tuple_size<T>::value in a SFINAE context must be a
// substitution failure for dynamic-size types, not a hard error.
template <typename T, typename = void>
struct is_tuple_like : std::false_type {};
template <typename T>
struct is_tuple_like<T, internal::void_t<decltype(std::tuple_size<T>::value)>> : std::true_type {};
template <typename T, typename = void>
struct has_tuple_element0 : std::false_type {};
template <typename T>
struct has_tuple_element0<T, internal::void_t<typename std::tuple_element<0, T>::type>> : std::true_type {};
void check_sfinae_friendly_detection() {
STATIC_CHECK((is_tuple_like<Vector3d>::value));
STATIC_CHECK((is_tuple_like<Matrix2f>::value));
STATIC_CHECK((is_tuple_like<Array3i>::value));
STATIC_CHECK((!is_tuple_like<VectorXd>::value));
STATIC_CHECK((!is_tuple_like<MatrixXf>::value));
STATIC_CHECK((!is_tuple_like<ArrayXd>::value));
STATIC_CHECK((!is_tuple_like<Matrix<double, 3, Dynamic>>::value));
STATIC_CHECK((!is_tuple_like<Matrix<double, Dynamic, 3>>::value));
STATIC_CHECK((has_tuple_element0<Vector3d>::value));
STATIC_CHECK((has_tuple_element0<Array3i>::value));
STATIC_CHECK((!has_tuple_element0<VectorXd>::value));
STATIC_CHECK((!has_tuple_element0<ArrayXXf>::value));
}
EIGEN_DECLARE_TEST(structured_bindings) {
CALL_SUBTEST_1(check_vector_bindings<double>());
CALL_SUBTEST_1(check_vector_bindings<float>());
@@ -200,6 +230,7 @@ EIGEN_DECLARE_TEST(structured_bindings) {
CALL_SUBTEST_4(check_matrix_bindings<int>());
CALL_SUBTEST_5(check_tuple_size());
CALL_SUBTEST_5(check_tuple_element());
CALL_SUBTEST_5(check_sfinae_friendly_detection());
CALL_SUBTEST_6(check_storage_order_semantics<double>());
CALL_SUBTEST_6(check_storage_order_semantics<int>());
}