[clang] [libcxx] [Cygwin][MinGW] Internal class in explicitly-instantiation-declarated template should be instantiated (PR #140145)

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jeremyd2019 wrote:

I've hacked up llvm-mingw workflow to build and test toolchain and libc++ for mingw at https://github.com/jeremyd2019/llvm-mingw/actions/runs/15075388808, hopefully I got the workflow right and this will be the correct test of af82a52986d549e32a1384630d4a26b8ecfa7687

https://github.com/llvm/llvm-project/pull/140145