Error "undefined method `by_macos?'" when running `brew link`

[waldyrious]

• 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)

I was trying to link brewed gettext to be able to compile the fntsample project. I installed all the dependencies listed in the project's README with Homebrew, but when I tried to configure it with the cmake command, I got the following error

CMake Error at /usr/local/Cellar/cmake/3.17.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:164 (message):
  Failed to find Gettext libintl (missing: Intl_INCLUDE_DIR)

To make gettext findable by CMake, I ran

brew link --force gettext

as was suggested in this comment which I found while searching for the CMake error message.

What happened (include command output)

I got the following error:

Command output

$ brew link --force gettext
Error: undefined method `by_macos?' for :shadowed_by_macos:Symbol
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/cmd/link.rb:71:in `block in link'
/usr/local/Homebrew/Library/Homebrew/cmd/link.rb:40:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/link.rb:40:in `link'
/usr/local/Homebrew/Library/Homebrew/brew.rb:110:in `
  

What you expected to happen

I expected the brewed gettext to be linked in /usr/local.

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

$ brew install gettext
$ brew link --force gettext

Output of brew config and brew doctor commands

$ brew config
HOMEBREW_VERSION: 2.2.12-56-g793407a
ORIGIN: https://github.com/Homebrew/brew
HEAD: 793407a2e080980636c3ec274429cff2e345672b
Last commit: 7 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: b86189eea34842c3867d3b5462a388536beb7be8
Core tap last commit: 9 minutes ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_BROWSER: open
HOMEBREW_EDITOR: micro
HOMEBREW_GITHUB_API_TOKEN: set
HOMEBREW_MAKE_JOBS: 8
CPU: octa-core 64-bit kabylake
Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
Clang: 11.0 build 1103
Git: 2.26.0 => /usr/local/bin/git
Curl: 7.64.1 => /usr/bin/curl
Java: 14
macOS: 10.15.3-x86_64
CLT: 11.4.0.0.1.1583798712
Xcode: N/A

$ brew doctor
Your system is ready to brew.

[issyl0]

Oops, thanks for the report! Should be fixed by #7337. Please try to brew update and brew link gettext again. You'll get a warning message, but at least it won't crash!