Describe the bug
In the implementation of std::basic_streambuf::seekpos(), the return statement constructs a value of type std::streampos(-1). This is only works correctly when pos_type is std::streampos. When pos_type is something else, such as std::fpos with a custom state type, the return statement generates a warning because the constructed value gets converted to pos_type via two user defined conversions (which is not allowed by standard C++). The resulting code does the right thing, but the warnings are annoying, and the implementation is clearly incorrect.

As far as I can tell, both return streamoff(-1) and return pos_type(-1) are proper ways to correct the return statement.

Another member function, std::basic_streambuf::seekoff(), suffers from the same problem.

Command-line test case

#include <string>
#include <streambuf>

struct State {};

struct CharTraits : std::char_traits<char> {
    using state_type = State;
    using pos_type = std::fpos<State>;
};

struct Streambuf : std::basic_streambuf<char, CharTraits> {};

https://gcc.godbolt.org/z/5b7Yfsefo

Expected behavior
No warnings should be generated.

STL version
git commit hash
https://github.com/microsoft/STL/commit/a9321cfe53ea31a7e197d5d8336167d6ca3de8b6

Additional context
DevCom-1495852