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• 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)

Create a formulae with the following url (from branch swift that has a top level folder also named swift:

url "https://github.com/zeroc-ice/ice.git", :tag => "swift"

What happened (include command output)

During a second run brew install (using the now cached repo) brew tires to update/reset the git branch and fails.

==> Cloning https://github.com/zeroc-ice/ice.git

Updating /Users/joe/Library/Caches/Homebrew/slice2swift--git

git config remote.origin.url https://github.com/zeroc-ice/ice.git

git config remote.origin.fetch +refs/tags/swift:refs/tags/swift

git config remote.origin.tagOpt --no-tags

==> Checking out tag swift

git checkout -f swift --

Already on 'swift'

git reset --hard swift

fatal: ambiguous argument 'swift': both revision and filename

Use '--' to separate paths from revisions, like this:

'git  [...] -- [...]'

Error: An exception occurred within a child process:

DownloadError: Failed to download resource "slice2swift"

Failure while executing; git reset --hard swift exited with 128. Here's the output:

fatal: ambiguous argument 'swift': both revision and filename

Use '--' to separate paths from revisions, like this:

'git  [...] -- [...]'
  

What you expected to happen

Formula will install

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

create formula with branch name XXX and top level folder in that repo with the same name XXX.
brew install
brew install a second time