macos

Merge pull request #198 from tcbrindle/pr/flatten_with #610

Job	Run time
build (LLVM-Clang-16, Headers, Debug)	58s
build (LLVM-Clang-16, Headers, Release)	1m 32s
build (LLVM-Clang-17, Headers, Debug)	1m 23s
build (LLVM-Clang-17, Headers, Release)	1m 32s
build (LLVM-Clang-17, Module, Debug)	1m 10s
build (LLVM-Clang-17, Module, Release)	1m 16s
build (LLVM-Clang-18, Headers, Debug)	1m 52s
build (LLVM-Clang-18, Headers, Release)	1m 42s
build (LLVM-Clang-18, Module, Debug)	1m 29s
build (LLVM-Clang-18, Module, Release)	1m 21s
	14m 15s

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