HomebrewでXcodeに依存するFormulaは、Xcode 10.2が入っていないmacOS Mojaveへインストールできなくなるらしい… https://github.com/Homebrew/brew/pull/6083

Require Xcode 10.2 on macOS 10.14 by MikeMcQuaid · Pull Request #...

Xcode >=10.2 is required to build all Swift formulae on macOS >=10.14.4. Rather than requiring per-formula workarounds (e.g. Homebrew/homebrew-core#39446) this provides a more stable ...