
This code originated in a slightly different form in 8e88b22fd1ec65a344ce6e4facd6dad4b415b2ad:
8e88b22fd1/Library/Homebrew/extend/ENV.rb (L30-L32)
Back then, MacOS.default_compiler could return nil, which meant
ENV.compiler could do the same. This code was carried forward as the
surrounding code changed. At this point it should be unreachable.