Represents an installed gem. This is used for dependency resolution.

Methods
#
D
Instance Public methods
<=>(other)

Installed sources sort before all other sources

Source: show 

# File lib/rubygems/source/installed.rb, line 13
def <=> other
  case other
  when Gem::Source::Lock,
       Gem::Source::Vendor then
    -1
  when Gem::Source::Installed then
    0
  when Gem::Source then
    1
  else
    nil
  end
end
download(spec, path)

We don't need to download an installed gem

Source: show 

# File lib/rubygems/source/installed.rb, line 30
def download spec, path
  nil
end