Build fails with libc++ 19 and later: std::basic_string<uint8_t> in streams.h #11

Release/include/cpprest/streams.h uses std::basic_string<uint8_t> in Value2StringFormatter<uint8_t>. The standard only defines std::char_traits for the character types, and libc++ 19 removed the generic std::char_traits template that made other types work. So the header no longer compiles with libc++ 19 or later. Current libstdc++ still accepts it.

The use is in a public header, so it breaks every consumer that includes cpprest/streams.h with libc++. That includes the default toolchain on macOS (Apple Clang), and Clang with -stdlib=libc++ elsewhere.

Proposed Fix

Base the fix on microsoft/cpprestsdk#1829, which defines cpprestsdk's own character traits for uint8_t and adjusts the tests. Do not use #1820, which specializes std::char_traits<uint8_t> in namespace std. The standard does not allow that for a type that the program does not define.

Add a CI job that builds with Clang and libc++ (or on macOS), so that this stays fixed.

References

• microsoft/cpprestsdk#1812 ("Fails to build on recent libc++ (20 tested)")
• microsoft/cpprestsdk#1829 and #1820, open upstream

Part of #15.