User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:41.0) Gecko/20100101 Firefox/41.0 Build ID: 20150522030225 Steps to reproduce: Build firefox with this mozconfig: . $topsrcdir/browser/config/mozconfig export CC=clang export CXX=clang++ mk_add_options MOZ_MAKE_FLAGS="-j6" mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/objdir-ff ac_add_options --host=x86_64-pc-mingw32 ac_add_options --target=x86_64-pc-mingw32 ac_add_options --enable-optimize ac_add_options --disable-debug Actual results: it doesn't build and show the error in the config.log file Expected results: build firefox without this error

Do you know where the -Wimplicit-int flag is coming from? I cannot find any mention of it in our source tree...

I don't know where this flag come from and i didn't add it in my CFLAGS

Hmm, I have no idea what might be happening... Sorry!

I think it's because the compiler doesn't find "confdefs.h" but how can i fix it ?

AFAIK that file is generated by configure, I don't think that is the source of the error. Here is a question: if you remove the host and target triplets from your mozconfig, would this check in the configure script pass? Also, may I ask which version of clang-cl you're using, and how you have built it?

I've already try to build it whithout host and target settings but it didn't work too. I'm not building with clang-cl but with clang and clang++

Oh, that is not supported at all in our build system. You should try clang-cl. Note that the clang-cl port is not stable yet, and you may hit issues with it. Please file those bugs and make them block bug 752004. Thanks!