<valarray>: `slice_array`/`gslice_array` incorrectly bind to function arguments of type `slice`/`gslice`

[statementreply]

Describe the bug

MSVC STL's slice_array and gslice_array derive from slice and gslice, respectively.

STL/stl/inc/valarray

Lines 1382 to 1384 in 47cd703

	// CLASS TEMPLATE slice_array
	template <class _Ty>
	class slice_array : public slice { // define a slice of a valarray

STL/stl/inc/valarray

Lines 1560 to 1562 in 47cd703

	// CLASS TEMPLATE gslice_array
	template <class _Ty>
	class gslice_array : public gslice { // define a generalized slice of a valarray

When a function overload set with an overload for both valarray<bool> and slice is called with a slice_array<bool> argument, the valarray<bool> overload should be selected (slice_array<bool> is implicitly convertible to valarray<bool>) according to the standard. However, with MSVC STL, the slice overload is incorrectly selected because it is the base class of slice_array<bool>, and derived-to-base conversion is better than user-defined conversion in overload resolution.

Command-line test case

D:\Temp>type slice.cpp

#include <cassert>
#include <valarray>
using namespace std;

int main() {
    valarray<int> a(10);
    valarray<bool> b(false, 20);
    valarray<int> c = a[b[slice(0, 10, 2)]];
    assert(c.size() == 0);
    return 0;
}

D:\Temp>cl /EHsc /W4 /WX slice.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29812 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

slice.cpp
Microsoft (R) Incremental Linker Version 14.28.29812.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:slice.exe
slice.obj

D:\Temp>.\slice.exe
Assertion failed: c.size() == 0, file slice.cpp, line 9

STL version

c7b1405

Additional context

This bug is discovered when reviewing #1627, where slice_array<int> == slice_array<int> incorrectly calls the newly added operator==(const slice&, const slice&).

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.