<valarray>: Inconsistent use of ADL

[cpplearner]

Describe the bug
Non-member functions in <valarray> are inconsistent about whether to use ADL.

abs enables ADL:

STL/stl/inc/valarray

Lines 749 to 752 in f9b1dcc

	template <class _Ty>
	_NODISCARD valarray<_Ty> abs(const valarray<_Ty>& _Left) {
	_VALOP(_Ty, _Left.size(), abs(_Left[_Idx]));
	}

Other functions disable ADL by qualify the function call with _CSTD. For example:

STL/stl/inc/valarray

Lines 834 to 837 in f9b1dcc

	template <class _Ty>
	_NODISCARD valarray<_Ty> sqrt(const valarray<_Ty>& _Left) {
	_VALOP(_Ty, _Left.size(), _CSTD sqrt(_Left[_Idx]));
	}

Both libstdc++ and libc++ use ADL for all these functions.

Command-line test case
STL version (git commit or Visual Studio version): 19.24.28218.2

D:\>type test.cpp
#include <valarray>

namespace ns {
    struct A {};

    A abs(A);
    A sqrt(A);
}

int main() {
    std::valarray<ns::A> a;
    (void)std::abs(a);
    (void)std::sqrt(a);
}

D:\>cl /c test.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.24.28218.2 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
D:\...\valarray(836): error C2665: 'sqrt': none of the 3 overloads could convert all the argument types
D:\...\cmath(485): note: could be 'long double sqrt(long double) noexcept'
D:\...\cmath(241): note: or       'float sqrt(float) noexcept'
D:\Windows Kits\10\include\10.0.17763.0\ucrt\corecrt_math.h(485): note: or       'double sqrt(double)'
D:\...\valarray(836): note: while trying to match the argument list '(const _Ty)'
        with
        [
            _Ty=ns::A
        ]
test.cpp(11): note: see reference to function template instantiation 'std::valarray<ns::A> std::sqrt<ns::A>(const std::valarray<ns::A> &)' being compiled

Expected behavior
No error.

Additional context
N/A

[miscco]

That seems to indeed be a bug. From valarray.transcend $1:

Requires: Each of these functions may only be instantiated for a type T to which a unique function with the indicated name can be applied (unqualified).
This function shall return a value which is of type T or which can be unambiguously implicitly converted to type T.

So each of those functions should be indeed called unqualified.