Extended Description

This bug has been encountered when testing MSVC STL shiny new constexpr string implementation with clang 12.x. A full reproducer is attached.

The code in question is:

#include <string>

constexpr bool test() {
    auto meow = "Hiss";
    [[maybe_unused]] auto it = std::string::const_iterator{meow + 4, nullptr} - 2;
    return true;
}

int main() {
    static_assert(test());
}

This explicitly creates a string iterator and call operator-. Now clangs gives the following error:

C:\STL\stl\inc\xstring(1964,16): warning: address of stack memory associated with local variable '_Tmp' returned
      [-Wreturn-stack-address]
        return _Tmp -= _Off;

This stems from the conventional implementation of operator+=

    _CONSTEX#20 _CONTAINER _String_const_iterator& operator+=(const difference_type _Off) noexcept {
#if _ITERATOR_DEBUG_LEVEL >= 1
        _Verify_offset(_Off);
#endif // _ITERATOR_DEBUG_LEVEL >= 1
        _Ptr += _Off;
        return *this;
    }

Here a reference is returned that seems to be mistaken of taking the address? The same problem happens with operator+=

Now the real fun part is, that I was fully unable to reproduce this in any meaningfull way. This is the furthest I came

#include <string>

constexpr bool test() {
    auto meow = "Hiss";
    [[maybe_unused]] auto it =
        std::_String_const_iterator<std::_String_val<std::_Simple_types<char>>>{meow + 4, nullptr} - 2;
    return true;
}

int main() {
    static_assert(test());
}

Even simply copying the content of _String_const_iterator into the file and using it instead of the real thing makes the bug go away. So there seems to be some highly unusual problem around