Default non-throw move ctor with std::unordered_map member is not move constructible

[cloudhan]

Command-line test case
STL version (git commit or Visual Studio version):

> cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.23.28105.4 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

Minimal reproducible test case:

#include <type_traits>
#include <unordered_map>
// #include <absl/container/flat_hash_map.h>

class MyStruct {
public:
    MyStruct() {}

    MyStruct(const MyStruct&) = delete;
    MyStruct(MyStruct&&) noexcept = default;
    MyStruct& operator=(const MyStruct&) = delete;
    MyStruct& operator=(MyStruct&&) = delete;

private:
    std::unordered_map<int, int> map_;
    // absl::flat_hash_map<int, int> map_;
};


int main() {
    static_assert(std::is_constructible<MyStruct>::value, "static_assert is_constructible");
    static_assert(std::is_nothrow_constructible<MyStruct>::value, "static_assert is_nothrow_constructible");
    static_assert(std::is_move_constructible<MyStruct>::value, "static_assert is_move_constructible");
    static_assert(std::is_nothrow_move_constructible<MyStruct>::value, "static_assert is_nothrow_move_constructible");
    static_assert(std::is_move_assignable<MyStruct>::value, "static_assert is_move_assignable");

    return 0;
}

compilation command:

> cl /IC:\tools\vcpkg\installed\x64-windows\include\ /std:c++17 /EHs .\noexcept.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.23.28105.4 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

noexcept.cpp
.\noexcept.cpp(22): error C2338: static_assert is_nothrow_constructible
.\noexcept.cpp(23): error C2338: static_assert is_move_constructible
.\noexcept.cpp(24): error C2338: static_assert is_nothrow_move_constructible
.\noexcept.cpp(25): error C2338: static_assert is_move_assignable

Expected behavior
In the above case, std::is_move_constructible and std::is_nothrow_move_constructible should be true, aka, only two static_assert fail, line 22, line 25.

Additional context

1. with abseil's flat_hash_map, it is indeed move constructible and nothrow move constructible.
2. with MyStruct(MyStruct&&) = default;, it is indeed move constructible but NOT nothrow move constructible.

[cloudhan]

FYI, why absl::flat_hash_map is used? Because this code snippet is reduce from a complex form of library code, and abseil lib is included.

It compiles with GCC but not MSVC. MSVC keeps complaining copy ctor is deleted when you have something like std::move(my_struct_object) where my_struct_object is of type MyStruct. The error message is quite misleading. It should at least directly point to the line MyStruct(MyStruct&&) noexcept = default;.

[cloudhan]

Similar issue https://stackoverflow.com/questions/47604029/move-constructors-of-stl-containers-in-msvc-2017-are-not-marked-as-noexcept

[CaseyCarter]

Expected behavior
In the above case, std::is_move_constructible and std::is_nothrow_move_constructible should be true

That is not Standard-mandated behavior. From the class template synopsis in [unord.map.overview], neither the default constructor nor move constructor is depicted as having a noexcept-specifier. Notably our implementation uses separately-allocated sentinel nodes which necessitates an allocation in both the default constructor and move constructor, so they cannot be noexcept.

[cloudhan]

OK, after reading the C++17 standard.

dcl.fct.def.default

3 If a function that is explicitly defaulted is declared with a noexcept-specifier that does not produce the same exception specification as the implicit declaration ([except.spec]), then
(3.1) if the function is explicitly defaulted on its first declaration, it is defined as deleted;
(3.2) otherwise, the program is ill-formed.
So it is indeed expected behavior.

Still hope you guys can improve the error message.

[CaseyCarter]

There isn't much of anything the STL can do about diagnostics that compilers choose to emit for code unrelated to the STL. I suggest that you file a suggestion at Developer Community about improving diagnostics for this case.