Probably need depends_on "expat" to avoid breaking installs on macOS 15. See also Homebrew/brew#19315

This is part of the odd situation of dependents of Python now propagating expat. Essentially will end up with dozens of formulae with brew expat linkage unless we handle this differently.

Some alternatives:

• [email protected]: bundle expat on Ventura/Sonoma #207887
• [email protected]: use newer expat.h on macOS 13.7.2 and 14.7.2 #207767
• Handle this in brew superenv and remove expat if only a dependency of [email protected]

Or we just accept expat usage everywhere.

Seems fine to accept it in dependents to me. There's already linkage anyway.

The formulae impacted (i.e. those with uses_from_macos "expat" and [email protected] in dependency tree) are:

1. aarch64-elf-gdb
2. afflib
3. arm-none-eabi-gdb
4. gdb ([email protected] for now due to gdbgui)
5. gdcm
6. graph-tool
7. i386-elf-gdb
8. opencolorio
9. riscv64-elf-gdb
10. votca
11. vtk
12. x86_64-elf-gdb

There were originally more before removing [email protected] from glib runtime dependencies.

One thing about current state is there is a higher risk of mixing expat libraries when installing a combination of Sequoia and Sonoma bottles, i.e. formula bottled/built on Sequoia may be linked to system expat but Sonoma bottle was linked to brew expat and then a project that combines these may crash from multiple expat.

Though we don't really care about this for ncurses and libxml2 dependents which has similar concerns.

Anyway, the existing vtk bottle is still linked to system expat so we shouldn't modify dependency until new bottle is needed:

❯ brew linkage vtk | grep expat
  /usr/lib/libexpat.1.dylib

Either on next version bump or revision bump like #207100