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release/21.x: [libc++] Fix std::variant evaluating template arguments too eagerly (#151028) #153064

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Merged
merged 1 commit into from
Aug 18, 2025
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release/21.x: [libc++] Fix std::variant evaluating template arguments too eagerly (#151028) #153064

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10 changes: 5 additions & 5 deletions libcxx/include/__type_traits/invoke.h
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Original file line number Diff line number Diff line change
Expand Up @@ -67,20 +67,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD

#if __has_builtin(__builtin_invoke)

template <class... _Args>
using __invoke_result_t _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));

template <class, class... _Args>
struct __invoke_result_impl {};

template <class... _Args>
struct __invoke_result_impl<__void_t<__invoke_result_t<_Args...> >, _Args...> {
using type _LIBCPP_NODEBUG = __invoke_result_t<_Args...>;
struct __invoke_result_impl<__void_t<decltype(__builtin_invoke(std::declval<_Args>()...))>, _Args...> {
using type _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));
};

template <class... _Args>
using __invoke_result _LIBCPP_NODEBUG = __invoke_result_impl<void, _Args...>;

template <class... _Args>
using __invoke_result_t _LIBCPP_NODEBUG = typename __invoke_result<_Args...>::type;

template <class... _Args>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __invoke_result_t<_Args...> __invoke(_Args&&... __args)
_NOEXCEPT_(noexcept(__builtin_invoke(std::forward<_Args>(__args)...))) {
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16 changes: 16 additions & 0 deletions libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
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Expand Up @@ -173,12 +173,28 @@ void test_vector_bool() {
assert(std::get<0>(v) == true);
}

struct ConvertibleFromAny {
template <class V>
ConvertibleFromAny(V) {}
};

int main(int, char**) {
test_T_ctor_basic();
test_T_ctor_noexcept();
test_T_ctor_sfinae();
test_no_narrowing_check_for_class_types();
test_construction_with_repeated_types();
test_vector_bool();

{ // Check that the constraints are evaluated lazily (see https://github.com/llvm/llvm-project/issues/151328)
struct Matcher {
Matcher() {}
Matcher(std::variant<ConvertibleFromAny>) {}
};

Matcher vec;
[[maybe_unused]] Matcher m = std::move(vec);
}

return 0;
}
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