Direct initialization with prvalue: Bug in MSVC?

Question

Consider the following code:

struct S
{
    S(int, double) {}
    explicit S(const S&) {}
    explicit S(S&&) {}
};

void i_take_an_S(S s) {}

S i_return_an_S() { return S{ 4, 2.0 }; }

int main()
{
    i_take_an_S(i_return_an_S());
}

With the '-std=c++17' flag, both g++ and clang++ compile this code just fine. MSVC (with the /std:c++17 flag), however, reports

"error C2664: 'void i_take_an_S(S)': cannot convert argument 1 from 'S' to 'S'"

as a compilation error, with the additional note

"Constructor for struct 'S' is declared 'explicit'".

According to C++17's initialization rules ( Explanation of point 3 ) S 's copy constructor should not be considered for the initialization of the S parameter of i_take_an_S ; S(int, double) should rather be selected as an exact match by direct-list-initialization.

Might this be a bug in MSVC?

Answer 1

Yes, MSVC seems to be wrong here.

Generally, since C++17, the initialization rules are so that S{ 4, 2.0 } will directly initialize the parameter S s of the function. (mandatory copy elision)

There is however an exception. An implementation is allowed to introduce a copy in a function parameter or a return value if the class type has only deleted or trivial copy/move constructors and destructor (and at least one of the former non-deleted).

That you declare the copy and move constructor explicit doesn't change that they are copy/move constructors. Because you are not using = default to define them, they are not trivial. Therefore the special permission does not apply and it is wrong of MSVC to try to perform a copy.

Furthermore this special kind of copy ignores accessibility and overload resolution and therefore explicit shouldn't be relevant even if it was performed, see [class.temporary]/3 .

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When exactly copy elision is performed affects the ABI however, so if this is a defect in MSVC's ABI, then it might not be easily fixed.

Answer 2

Might this be a bug in MSVC?

Yes, MSVC is wrong in rejecting the code.

The operand S{ 4, 2.0 } of the return statement return S{ 4, 2.0 } is a prvalue of type S and from C++17 onwards due mandatory copy elison , the parameter named s of function i_take_an_S is constructed directly (without using any copy/move ctor) from S{ 4, 2.0 } .

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This can be seen from copy elison :

Under the following circumstances, the compilers are required to omit the copy and move construction of class objects, even if the copy/move constructor and the destructor have observable side-effects. T he objects are constructed directly into the storage where they would otherwise be copied/moved to . The copy/move constructors need not be present or accessible:

In a return statement, when the operand is a prvalue of the same class type (ignoring cv-qualification) as the function return type:

 T f() { return T(); } f(); // only one call to default constructor of T