<complex>: invalid conversions allowed

[fsb4000]

Describe the bug
Our inheritance implementation is allowing this to compile when it shouldn't.

Command-line test case

C:\Temp>type main.cpp
#include <complex>
#include <cassert>

int main ()
{
    const std::complex<long double> cd(2.5, 3.5);
    std::complex<float> cf = cd;
    assert(cf.real() == cd.real());
    assert(cf.imag() == cd.imag());
}

C:\Temp>cl /EHsc /W4 /WX main.cpp
Оптимизирующий компилятор Microsoft (R) C/C++ версии 19.28.29213 для x64
(C) Корпорация Майкрософт (Microsoft Corporation).  Все права защищены.

main.cpp
Microsoft (R) Incremental Linker Version 14.28.29213.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

C:\Temp>main.exe

C:\Temp>type main.cpp
#include <complex>
#include <cassert>

int main ()
{
    const std::complex<double> cd(2.5, 3.5);
    std::complex<float> cf = cd;
    assert(cf.real() == cd.real());
    assert(cf.imag() == cd.imag());
}

C:\Temp>cl /EHsc /W4 /WX main.cpp
Оптимизирующий компилятор Microsoft (R) C/C++ версии 19.28.29213 для x64
(C) Корпорация Майкрософт (Microsoft Corporation).  Все права защищены.

main.cpp
Microsoft (R) Incremental Linker Version 14.28.29213.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

C:\Temp>main.exe

C:\Temp>type main.cpp
#include <complex>
#include <cassert>

int main ()
{
    const std::complex<long double> cd(2.5, 3.5);
    std::complex<double> cf = cd;
    assert(cf.real() == cd.real());
    assert(cf.imag() == cd.imag());
}

C:\Temp>cl /EHsc /W4 /WX main.cpp
Оптимизирующий компилятор Microsoft (R) C/C++ версии 19.28.29213 для x64
(C) Корпорация Майкрософт (Microsoft Corporation).  Все права защищены.

main.cpp
Microsoft (R) Incremental Linker Version 14.28.29213.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

C:\Temp>main.exe

Expected behavior
Compile time error

STL version
Microsoft Visual Studio Community 2019 Preview Version 16.8.0 Preview 2.0

Additional context
Skipped libcxx test

STL/tests/libcxx/expected_results.txt

Lines 557 to 560 in 06827fe

	# STL bug: Our inheritance implementation is allowing this to compile when it shouldn't.
	std/numerics/complex.number/complex.special/double_long_double_implicit.compile.fail.cpp FAIL
	std/numerics/complex.number/complex.special/float_double_implicit.compile.fail.cpp FAIL
	std/numerics/complex.number/complex.special/float_long_double_implicit.compile.fail.cpp FAIL

[MattStephanson]

Besides the inheritance structure, there's the presence of additional non-explicit constructors:

STL/stl/inc/complex

Lines 794 to 805 in be0441c

	constexpr complex(const complex<float>&); // defined below
	constexpr explicit complex(const complex<long double>&); // defined below
	constexpr complex(const _Ty& _Realval = 0, const _Ty& _Imagval = 0)
	: _Complex_base<double, _Dcomplex_value>(_Realval, _Imagval) {}
	constexpr complex(const _Dcomplex_value& _Right)
	: _Complex_base<double, _Dcomplex_value>(_Right._Val[_RE], _Right._Val[_IM]) {}
	constexpr complex(const _Lcomplex_value& _Right)
	: _Complex_base<double, _Dcomplex_value>(
	static_cast<double>(_Right._Val[_RE]), static_cast<double>(_Right._Val[_IM])) {}

I guess these exist for compatibility with the CRT complex types, but it seems strange that they only exist for the same or larger types. The most parsimonious fix would be to make the appropriate constructors explicit, but maybe there's a case for either removing them altogether or adding constructors for the narrower CRT types too. Thoughts?

[StephanTLavavej]

Great catch, @MattStephanson - @Arzaghi's PR removes them entirely, since they are completely unused and I don't believe we ever documented them.