This is used to indicate a formula is a version of another formula.
This will be used to provide a consistent interface for older formulae
versions and replaces the use of `conflicts_with`.
If you specify a formula more than once or it exists in the Cellar with
an alias name and the main name (e.g. `qt` and `qt5`) you can see the
same formula showing up more than once. Instead, resolve these output
lists of formulae such that they are unique based on their `name`. This
doesn't use `full_name` as it's `name` that's use for the `Cellar`.
Return `opt_prefix` if it exists and `prefix` is not called from within
the same formula's `install` or `post_install` methods. Otherwise, fall
back to the existing functionality.
This avoids the need to use `opt_prefix` etc. everywhere and generally
means we don't expose an implementation detail (i.e. the full Cellar
path) to dependents that have a habit of hard-coding it.
Fish shell allows third-party software vendors to put their own function files in a directory for their software.
For brew installed Fish shell, this is /usr/local/share/fish/vendor_functions.d
* `Formula#linked?` returns true if formula linked
* `Formula#optlinked?` returns true if formula linked to opt
formula installed to the Cellar
* `Formula#prefix_linked?` returns true if linked keg points to the prefix
passed as an argument
* `Formula#linked_version` to get linked version of the formula
* Causes a bug in Formula#installed_alias_target_changed? when
Formula#superseds_an_installed_formula? returns true because
Formula#old_installed_formulae includes f for some Formula f.
* Causes a bug when foo@2.4 with alias foo has HEAD or devel version and
we try to `brew upgrade foo --devel|--HEAD` from stable. The upgrade fails
while since we installing formula to the same prefix it's alredy installed.
The reason for that is that we use
`formula_to_install = outdated.map(&:latest_formula)` in cmd/upgrade
before calling upgrade_formula on foo.
```ruby
def latest_formula
installed_alias_target_changed? ? current_installed_alias_target : self
end
```
Formula#installed_alias_target_changed? compares formulae using
Formula#==, which is wrong for this case, thus Formula#latest_formula doesn't
return self and returns Formula#current_installed_alias_target with spec
foo was initially installed instead of devel or HEAD, causing the error.
