<regex>: Error C2664 in std::regex_traits::transform

[AlexGuteniev]

Describe the bug
std::regex_traits::transform tries to use const _Elem* as internal iterator instead of user-defined iterator.

Command-line test case

d:\Temp2>type repro.cpp
#include <regex>
#include <iostream>

template <class It>
class forward_iterator
{
    It it_;

    template <class U> friend class forward_iterator;
public:
    typedef          std::forward_iterator_tag                 iterator_category;
    typedef typename std::iterator_traits<It>::value_type      value_type;
    typedef typename std::iterator_traits<It>::difference_type difference_type;
    typedef It                                                 pointer;
    typedef typename std::iterator_traits<It>::reference       reference;

    It base() const { return it_; }

    forward_iterator() : it_() {}
    explicit forward_iterator(It it) : it_(it) {}
    template <class U>
    forward_iterator(const forward_iterator<U>& u) : it_(u.it_) {}

    reference operator*() const { return *it_; }
    pointer operator->() const { return it_; }

    forward_iterator& operator++() { ++it_; return *this; }
    forward_iterator operator++(int)
    {
        forward_iterator tmp(*this); ++(*this); return tmp;
    }

    friend bool operator==(const forward_iterator& x, const forward_iterator& y)
    {
        return x.it_ == y.it_;
    }
    friend bool operator!=(const forward_iterator& x, const forward_iterator& y)
    {
        return !(x == y);
    }

    template <class T>
    void operator,(T const &) = delete;
};

int main()
{
    std::regex_traits<char> t;
    const char a[] = "a";
    const char B[] = "B";
    typedef forward_iterator<const char*> F;
    bool r = t.transform(F(a), F(a + 1)) > t.transform(F(B), F(B + 1));
    (void) r;
}

d:\Temp2>cl /EHsc /W4 .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\regex(283): error C2664: 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::collate<_Elem>::transform(const _Elem *,const _Elem *) const': cannot convert argument 1 from '_FwdIt' to 'const _Elem *'
        with
        [
            _Elem=char
        ]
        and
        [
            _FwdIt=F
        ]
        and
        [
            _Elem=char
        ]
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\regex(283): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\locale(41): note: see declaration of 'std::collate<_Elem>::transform'
        with
        [
            _Elem=char
        ]
.\repro.cpp(52): note: see reference to function template instantiation 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::_Regex_traits<char>::transform<F>(_FwdIt,_FwdIt) const' being compiled
        with
        [
            _FwdIt=F
        ]
.\repro.cpp(52): note: see reference to function template instantiation 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::_Regex_traits<char>::transform<F>(_FwdIt,_FwdIt) const' being compiled
        with
        [
            _FwdIt=F
        ]

Expected behavior
The program should compile

STL version

Microsoft Visual Studio Professional 2019 Preview
Version 16.7.0 Preview 3.1

Additional context
The following is the equivalent of the repro:

STL/tests/libcxx/expected_results.txt

Lines 564 to 565 in 5be7d49

	# STL bug: regex_traits::transform() isn't following the Standard.
	std/re/re.traits/transform.pass.cpp FAIL

Also tracked by DevCom-1076807 and Microsoft-internal VSO-1142061 / AB#1142061.