传递指针参数时意外的模板实例化

Question

以下代码无法编译（在 godbolt.org 上使用带有 /std:c++latest 的 MSVC 19.28 和带有 -std=c++20 的 Clang 12.0 进行测试）：

#include <variant>

struct some_thing;
using var = std::variant<some_thing>;

// There should be NO need to instantiate the template here, right?
void take(var* v) {}

int main() {
    var* v = nullptr;
    take(v); // This is the offending line - tries to instantiate the variant template and fails as 'some_thing' is undefined up until now.
    return 0;
}

对 main 的调用如何需要模板实例化？我们只是传递一个指针。这是一个错误还是我错过了什么？

模板实例，如果有一个指针类型的模板的工作（如我做的话），而不是时，不应发生typedef'ing它的typedef不引入新的类型（即模板实例）。我可能仍然缺少一些东西，但这应该使我的示例代码能够编译。

请参阅下面的 MSVC 错误消息。请注意，此问题与理解错误消息无关。它是关于为什么甚至首先发生模板实例化。

C:/data/msvc/14.28.29333/include\type_traits(807): error C2139: 'some_thing': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_trivially_destructible'
<source>(3): note: see declaration of 'some_thing'
C:/data/msvc/14.28.29333/include\variant(956): note: see reference to class template instantiation 'std::is_trivially_destructible<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(956): note: see reference to variable template 'const bool conjunction_v<std::is_trivially_destructible<some_thing> >' being compiled
C:/data/msvc/14.28.29333/include\variant(1014): note: see reference to alias template instantiation 'std::_Variant_destroy_layer<some_thing>' being compiled
<source>(11): note: see reference to class template instantiation 'std::variant<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\type_traits(661): error C2139: 'some_thing': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_constructible'
<source>(3): note: see declaration of 'some_thing'
C:/data/msvc/14.28.29333/include\xsmf_control.h(61): note: see reference to class template instantiation 'std::is_move_constructible<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(62): note: see reference to variable template 'const bool conjunction_v<std::is_move_constructible<some_thing>,std::negation<std::conjunction<std::is_trivially_move_constructible<some_thing> > > >' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(100): note: see reference to alias template instantiation 'std::_SMF_control_move<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(139): note: see reference to alias template instantiation 'std::_SMF_control_copy_assign<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(146): note: see reference to alias template instantiation 'std::_SMF_control_move_assign<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(1014): note: see reference to alias template instantiation 'std::_SMF_control<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\type_traits(630): error C2139: 'some_thing': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_constructible'
<source>(3): note: see declaration of 'some_thing'
C:/data/msvc/14.28.29333/include\xsmf_control.h(39): note: see reference to class template instantiation 'std::is_copy_constructible<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(40): note: see reference to variable template 'const bool conjunction_v<std::is_copy_constructible<some_thing>,std::negation<std::conjunction<std::is_trivially_copy_constructible<some_thing> > > >' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(63): note: see reference to alias template instantiation 'std::_SMF_control_copy<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(491): error C2079: 'std::_Variant_storage_<false,some_thing>::_Head' uses undefined struct 'some_thing'
C:/data/msvc/14.28.29333/include\variant(838): note: see reference to class template instantiation 'std::_Variant_storage_<false,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(941): note: see reference to class template instantiation 'std::_Variant_base<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(85): note: see reference to class template instantiation 'std::_Variant_destroy_layer_<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\xsmf_control.h(124): note: see reference to class template instantiation 'std::_Deleted_copy_assign<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(1014): note: see reference to class template instantiation 'std::_Deleted_move_assign<std::_Variant_destroy_layer_<some_thing>,some_thing>' being compiled
C:/data/msvc/14.28.29333/include\type_traits(730): error C2139: 'some_thing': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_destructible'
<source>(3): note: see declaration of 'some_thing'
C:/data/msvc/14.28.29333/include\variant(1016): note: see reference to class template instantiation 'std::is_destructible<some_thing>' being compiled
C:/data/msvc/14.28.29333/include\variant(1016): note: see reference to variable template 'const bool conjunction_v<std::is_object<some_thing>,std::negation<std::is_array<some_thing> >,std::is_destructible<some_thing> >' being compiled
C:/data/msvc/14.28.29333/include\variant(1016): error C2338: variant<Types...> requires all of the Types to meet the Cpp17Destructible requirements N4828 [variant.variant]/2.
Compiler returned: 2

Answer 1

您的问题与ADL有关。

从链接：

否则，对于函数调用表达式中的每个参数，都会检查其类型以确定它将添加到查找中的关联的命名空间和类集。

检查将导致函数调用中使用的模板的实例化，如果在这种情况下std::variant需要 的类型定义，则会失败some_thing。

正如我们所知，ADL 是问题所在，在这种情况下，如果我们不想在其他命名空间或作用域中查找以take()查找std::variant<some_thing>使用:: operator.

如果您通过将格式不正确的行替换为::take(v);代码编译来更改代码，则 ADL 将不再发生。