[libcxx-commits] [libcxx] Fix __split_buffer_pointer_layout compatibility with swift's C++ interop (PR #170957)

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junov-google wrote:

> std::__split_buffer_pointer_layout<void, int, std::allocator<int>&> sb;
> // expected-error@*:*{{no matching constructor for initialization of 'std::__split_buffer_pointer_layout<void, int, std::allocator<int>&>'}}
> // expected-note@*:*{{candidate constructor not viable: constraints not satisfied}}
> // expected-note@*:*{{because '!is_reference_v<allocator_type>' evaluated to false}}
> // expected-note N {{.*}}

This is exactly the kind of test I was trying to write but I wasn't able to get it to detect the the change from this fix.  The `expected-error` part works fine, but that part passes with and without this fix.  The `expected-note` expectations don't work.  I'm guessing there a special incantation of `// RUN: ` that will make this work...  I'll keep trying.

https://github.com/llvm/llvm-project/pull/170957