Add syntax sugar for MPIDependency

Closes Homebrew/homebrew#20797.

Library/Homebrew/cmd/audit.rb

@@ -166,9 +166,9 @@ class FormulaAuditor
       when 'open-mpi', 'mpich2'
         problem <<-EOS.undent
           There are multiple conflicting ways to install MPI. Use an MPIDependency:
-            depends_on MPIDependency.new(<lang list>)
+            depends_on :mpi => [<lang list>]
           Where <lang list> is a comma delimited list that can include:
-            :cc, :cxx, :f90, :f77
+            :cc, :cxx, :f77, :f90
           EOS
       end
     end

Library/Homebrew/dependency_collector.rb

@@ -94,6 +94,7 @@ class DependencyCollector
     when :mysql      then MysqlDependency.new(tags)
     when :postgresql then PostgresqlDependency.new(tags)
     when :fortran    then FortranDependency.new(tags)
+    when :mpi        then MPIDependency.new(*tags)
     when :tex        then TeXDependency.new(tags)
     when :clt        then CLTDependency.new(tags)
     when :arch       then ArchRequirement.new(tags)

Library/Homebrew/requirements/mpi_dependency.rb

@@ -13,11 +13,14 @@ class MPIDependency < Requirement
 
   env :userpaths
 
-  def initialize *lang_list
-    @lang_list = lang_list
+  # This method must accept varargs rather than an array for
+  # backwards compatibility with formulae that call it directly.
+  def initialize(*tags)
     @non_functional = []
     @unknown_langs = []
-    super()
+    @lang_list = [:cc, :cxx, :f77, :f90] & tags
+    tags -= @lang_list
+    super(tags)
   end
 
   def mpi_wrapper_works? compiler

Library/Homebrew/test/test_mpi_dependency.rb

@@ -0,0 +1,12 @@
+require 'testing_env'
+require 'requirements/mpi_dependency'
+
+class MPIDependencyTests < Test::Unit::TestCase
+  def test_initialize_untangles_tags_and_wrapper_symbols
+    wrappers = [:cc, :cxx, :f77]
+    tags = [:optional, 'some-other-tag']
+    dep = MPIDependency.new(*wrappers + tags)
+    assert_equal wrappers, dep.lang_list
+    assert_equal tags, dep.tags
+  end
+end