Allow requirements to specify a default formula.

This allows default resolution of requirements without user intervention.

Closes Homebrew/homebrew#19627.

Library/Homebrew/requirement.rb

@@ -75,6 +75,11 @@ class Requirement
   end
 
   class << self
+    # The default formula to install to satisfy this requirement.
+    def default_formula(val=nil)
+      val.nil? ? @default_formula : @default_formula = val.to_s
+    end
+
     def fatal(val=nil)
       val.nil? ? @fatal : @fatal = val
     end
@@ -138,6 +143,13 @@ class Requirement
 
         next if prune
 
+        # TODO: Do this in a cleaner way, perhaps with another type of
+        # dependency type.
+        if req.class.default_formula
+          dependent.class.depends_on(req.class.default_formula)
+          next
+        end
+
         reqs << req
       end
     end

Library/Homebrew/requirements.rb

@@ -19,6 +19,7 @@ end
 
 class MysqlDependency < Requirement
   fatal true
+  default_formula 'mysql'
 
   satisfy { which 'mysql_config' }
 
@@ -40,6 +41,7 @@ end
 
 class PostgresqlDependency < Requirement
   fatal true
+  default_formula 'postgres'
 
   satisfy { which 'pg_config' }
 
@@ -110,6 +112,7 @@ end
 
 class MercurialDependency < Requirement
   fatal true
+  default_formula 'mercurial'
 
   satisfy { which('hg') }
 

Library/Homebrew/requirements/mpi_dependency.rb

@@ -9,6 +9,8 @@ class MPIDependency < Requirement
 
   fatal true
 
+  default_formula 'open-mpi'
+
   env :userpaths
 
   def initialize *lang_list