I am currently porting an experimental language to Windows. This experimental language was built in C++ with LLVM, and relies heavily on GCC extensions like VLAs and Compound Statement Expressions, which basically made it impossible to build with MSVC (although I have a truly horrifying idea I may attempt later). Luckily, you can now build things on Windows with Clang, which solves a lot of problems. However, clang-cl simply compiles the code - it still uses the Microsoft C++ headers and links to the Microsoft C++ runtime.