<algorithm>: fill fails to compile with volatile pointer inputs

[Ryan-rsm-McKenzie]

Describe the bug
Passing volatile pointers to fill with arguments that satisfy _Fill_memset_is_safe will fail to compile, as that branch calls memset which cannot take volatile pointers.

Command-line test case

E:\Temp>type repro.cpp
#include <cstdlib>
#include <iostream>
#include <algorithm>

void memzero(void* a_dst, std::size_t a_size)
{
    const auto     beg = static_cast<volatile char*>(a_dst);
    const auto     end = static_cast<volatile char*>(a_dst) + a_size;
    constexpr char val = 0;
    std::fill(beg, end, val);
}

int main()
{
    int num = 42;

    std::cout << num << '\n';
    memzero(&num, sizeof(num));
    std::cout << num << '\n';
}

E:\Temp>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29115 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.28.29115\include\xutility(4717): error C2664: 'void *memset(void *,int,size_t)': cannot convert argument 1 from '_FwdIt' to 'void *'
        with
        [
            _FwdIt=volatile char *
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.28.29115\include\xutility(4717): note: Conversion loses qualifiers
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.28.29115\include\vcruntime_string.h(63): note: see declaration of 'memset'
.\repro.cpp(10): note: see reference to function template instantiation 'void std::fill<volatile char*,char>(const _FwdIt,const _FwdIt,const _Ty &)' being compiled
        with
        [
            _FwdIt=volatile char *,
            _Ty=char
        ]

Expected behavior
The program should compile.

STL version

Microsoft Visual Studio Community 2019 Preview
Version 16.8.0 Preview 1.0

Additional context
Both GCC and Clang can compile this example, here's a godbolt link.

It will actually compile if you use std::fill(beg, end, 0); presumably because _Ty is now int and _Fill_memset_is_safe evaluates to false.

[AlexGuteniev]

@AdamBucior , since it overlaps with your PR on review, thoughts?

My understanding is that calling memset is a valid optimization for non-volatile pointers.
But when pointers are volatile, should keep calling loop version.
Should not cast away volatile and call memset, it is not safe.

[BillyONeal]

We should make this compile but I note that there's nothing useful you get from volatile here as far as the standard is concerned.

[AlexGuteniev]

@BillyONeal , I think it can implement SecureZeroMemory, no?

[AdamBucior]

Implemented an easy fix in #1160.

[AdamBucior]

Should it have some test coverage?

[AlexGuteniev]

Think so, as this is highly unusual, possibly in a separate test.

[AdamBucior]

Separate test is definitely too much. I would add it to std/tests/VSO_0180469_fill_family if it even needs test coverage.

[AdamBucior]

Added a simple test case.