MSVC C ++ ADL错误？

Question

This code works fine with GCC and Clang. It works fine in MSVC when using a custom type instead of std::chrono::duration. It works fine when using an operator+ over operator* . It works fine with MSVC pre 2018 instead of 2017/2015.

Am I missing anything obvious, or is this just a bug in MSVC? https://godbolt.org/z/EUWV7e

For completeness, here is the test case from the above link:

#include <chrono>

namespace A {
    class Foo {
    public:
        int mCount;
        constexpr explicit Foo(int count) : mCount( count ) {}
    };

    template<class Rep, class Period>
    inline Foo
    operator*(const Foo foo1, const std::chrono::duration<Rep, Period> duration) {
        return Foo(foo1.mCount * duration.count());
    }

    // For testing purposes, this is identical to operator* above.
    template<class Rep, class Period>
    inline Foo
    operator+(const Foo foo1, const std::chrono::duration<Rep, Period> duration) {
        return Foo(foo1.mCount * duration.count());
    }
}

int main() {
    A::Foo foo1(50);

    // This fails to compile for some reason?  Changing the '*' to a '+' works fine however.
    auto foo2 = foo1 * std::chrono::minutes(15);
    return foo2.mCount;
}

Answer 1

from <chrono> :

template<class _Rep1,
    class _Rep2,
    class _Period2> inline
    constexpr typename enable_if<is_convertible<_Rep1,
        typename common_type<_Rep1, _Rep2>::type>::value,
        duration<typename common_type<_Rep1, _Rep2>::type, _Period2> >::type
        operator*(
            const _Rep1& _Left,
            const duration<_Rep2, _Period2>& _Right) ...

Note that since std::chrono::minutes is duration<int, ratio<60>> -- _Rep2 is int . Now common_type<_Rep1, _Rep2> expands to (see <type_traits> ):

    struct common_type  // <Foo, int>
    {   // type is common type of Foo and int for two arguments
        typedef typename decay<
            decltype(false ? declval<Foo>() : declval<int>())
        >::type type;
    };

Error you observe is conditional operator complaint that somehow isn't "intercepted" by SFINAE. If you remove explicit from Foo's ctor -- it'll go away.

I am not intimately familiar with finer points of SFINAE behavior, but cppreference.com has curious note:

Only the failures in the types and expressions in the immediate context of the function type or its template parameter types or its explicit specifier (since C++20) are SFINAE errors. If the evaluation of a substituted type/expression causes a side-effect such as instantiation of some template specialization, generation of an implicitly-defined member function, etc, errors in those side-effects are treated as hard errors.

I am not sure this applies to your case... If it does -- then MS compiler is right, but their std lib has a problem. If it doesn't -- then it is probably an issue in compiler.

Edit: apparently MS had problems with SFINAE for a while...

Answer 2

I've reported this to the MSVC team at https://developercommunity.visualstudio.com/content/problem/381899/adl-sfinae-bug-in-mvc20152017-when-using-stdchrono.html and they've confirmed it is a bug in MSVC, and has been fixed in the latest release of MSVC2017. Thanks for the discussions!