[libcxx-commits] [libcxx] [libc++][modules] Uses _LIBCPP_USING_IF_EXISTS. (PR #90409)

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EricWF wrote:

> This attribute is used in the headers. Not using this in the modules has led to several issues. Add them to the modules to avoid these errors in other placed.

If  they're not needed without modules, then I suspect this is just papering over the root cause of the breakages.
Could you point me towards some concrete examples?


https://github.com/llvm/llvm-project/pull/90409