beanz added a comment.

This patch doesn't make sense to me.

In an in-tree build `CMAKE_SOURCE_DIR` would be the LLVM source directory which 
shouldn't match your `CMAKE_BINARY_DIR`.

In an out-of-tree build `CMAKE_SOURCE_DIR` would match the libcxx sure 
directory, which shouldn't match `CMAKE_BINARY_DIR`.

In an out-of-tree build `CMAKE_SOURCE_DIR` and `CMAKE_CURRENT_SOURCE_DIR`, and 
for an in-tree build (where they would be different) there should be no 
situation where the `CMAKE_CURRENT_SOURCE_DIR` could possibly be equal to the 
current source directory unless the user does something *really* strange.

By *really* strange I mean a workflow that involved checking out LLVM, and 
libcxx, then configuring LLVM from inside the libcxx directory. If we're 
looking to capture that kind of situation (configuring a build inside an 
arbitrary directory in the source tree) our existing mechanisms are 
insufficient in a great many ways, so I don't think we should go down that path.

If you think this patch is important can you please explain the specific cases 
where this catches errors that the current code doesn't?


https://reviews.llvm.org/D26375



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