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• are reporting a bug others will be able to reproduce and not asking a question. If you're not sure or want to ask a question do so on our Discourse: https://discourse.brew.sh
• ran a brew command and reproduced the problem with multiple formulae? If it's a problem with a single, official formula (not cask) please file this issue at Homebrew/homebrew-core: https://github.com/Homebrew/homebrew-core/issues/new/choose. If it's a brew cask problem please file this issue at https://github.com/Homebrew/homebrew-cask/issues/new/choose. If it's a tap (e.g. Homebrew/homebrew-php) problem please file this issue at the tap.
• ran brew update and can still reproduce the problem?
• ran brew doctor, fixed all issues and can still reproduce the problem?
• ran brew config and brew doctor and included their output with your issue?

What you were trying to do (and why)

When running brew upgrade, the fish formula was upgraded from 3.0.0 to 3.0.1.

What happened (include command output)

During the upgrade, this confusing error was raised. This left fish in a half-broken state, where it was installed but not linked.

==> Upgrading fish
==> Downloading https://homebrew.bintray.com/bottles/fish-3.0.1.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Pouring fish-3.0.1.mojave.bottle.tar.gz
Error: Cask 'fish' definition is invalid: invalid 'depends_on macos' value: ":snow_leopard"
  

What you expected to happen

Installation succeeds.

Step-by-step reproduction instructions (by running brew commands)

1. brew install fish

This is caused by 6f184ea, which rescues Cask::CaskUnavailableError but not Cask::CaskInvalidError. In this case the fish cask is invalid and can neither be installed nor loaded; this meant that brew install fish also fails, since it now depends on the cask being valid.

The second issue this exposes is that we should be able to handle this kind of exception in formula_installer a bit more graciously; it shouldn't leave a package installed but unlinked.