Please consider merging https://reviews.llvm.org/rL321777 into Clang 6, in order to fix https://bugs.llvm.org/show_bug.cgi?id=35028 "Clang does not use the strictest alignas in templates". MSVC's STL contains a workaround for this bug in its <execution> header. It would be nice, but not critical, to be able to remove this workaround.
Merged in r322676.