Both GCC for mingw target and clang in clang-cl more support implicit extern "C" for windows-specific entry points like WinMain, wWinMain, wmain, DllMain. Sadly, clang for mingw target does not support it yet. Ideally, it should be fixed in clang itself, but I could easily work around it by adding a few explicit extern "C" attributes.

Comment on attachment 8988181 [details] Bug 1471592 - Explicitly mark non-main win32 entry points as extern "C". https://reviewboard.mozilla.org/r/253414/#review260354 Sure, why not? Can you file a followup bug with clang to add implicit wmain and friends?

Sure, filled upstream bug: https://bugs.llvm.org/show_bug.cgi?id=38124

Comment on attachment 8988181 [details] Bug 1471592 - Explicitly mark non-main win32 entry points as extern "C". [Approval Request Comment] This is one of several MinGW Build patches I'd like to land in esr60 for Tor. It will prevent them from carrying their own patches for the lifetime of esr60 and will enable us to keep the MinGW build functioning and know if/when/how it was broken by new commits into esr60. This commit affects all configurations; but if it were broken the browser build jobs would crash often and loudly.

Martin fixed it in clang, so I think we'd better bump LLVM version instead of taking this patch.

What should we do for ESR60 at this point?