<complex>: pow(complex<float>, int) returns complex<float> instead of complex<double>

[amyw-msft]

The Standard states that pow(complex<float>, int) should treat both arguments as complex<double>, indicating that it should also return complex<double>. Instead it returns a complex<float>.

> type test.cpp

#include <complex>
#include <type_traits>

using namespace std;

int main()
{
    static_assert(
        is_same_v<
            complex<double>,
            decltype(pow(complex<float>{}, int{}))
        >,
        "pow(complex<float>, int) should return complex<double>"
        );
}

> cl /EHsc /nologo /W4 /std:c++17 test.cpp
test.cpp
test.cpp(12): error C2338: pow(complex<float>, int) should return complex<double>

WG21-N4835 [cmplx.over]/3 states:

Function template pow shall have additional overloads sufficient to ensure, for a call with at least one argument of type complex<T>:

• If either argument has type complex<long double> or type long double, then both arguments are effectively cast to complex<long double>.
• Otherwise, if either argument has type complex<double>, double, or an integer type, then both arguments are effectively cast to complex<double>.
• Otherwise, if either argument has type complex<float> or float, then both arguments are effectively cast to complex<float>.

libc++ will return the correct type. libstdc++ will not. https://godbolt.org/z/_TscRV

Also tracked by Microsoft-internal VSO-177118.

[Neargye]

It's reproduced only when the int is on the right, because there is a special overload.

STL/stl/inc/complex

Line 1588 in 3b0a1c9

_NODISCARD complex<_Ty> pow(const complex<_Ty>& _Left, int _Right) {

[Neargye]

Try to fix during #383