Allow requirements to specify a default formula.

This allows default resolution of requirements without user intervention.

Closes #19627.

8 files changed, 22 insertions, 0 deletions

diff --git a/Library/Formula/elixir.rb b/Library/Formula/elixir.rb
index 74b133d91..c129bac14 100644
--- a/Library/Formula/elixir.rb
+++ b/Library/Formula/elixir.rb
@@ -2,6 +2,7 @@ require 'formula'
 
 class ErlangInstalled < Requirement
   fatal true
+  default_formula 'erlang'
 
   satisfy { which 'erl' }
 
diff --git a/Library/Formula/mu.rb b/Library/Formula/mu.rb
index 9b50c5ea3..2d9d18f9a 100644
--- a/Library/Formula/mu.rb
+++ b/Library/Formula/mu.rb
@@ -3,6 +3,7 @@ require 'formula'
 class Emacs23Installed < Requirement
   fatal true
   env :userpaths
+  default_formula 'emacs'
 
   satisfy do
     `emacs --version 2>/dev/null` =~ /^GNU Emacs (\d{2})/
diff --git a/Library/Formula/mysql-connector-odbc.rb b/Library/Formula/mysql-connector-odbc.rb
index 06a7ed6d9..6bbbe3a7c 100644
--- a/Library/Formula/mysql-connector-odbc.rb
+++ b/Library/Formula/mysql-connector-odbc.rb
@@ -2,6 +2,7 @@ require 'formula'
 
 class MySqlInstalled < Requirement
   fatal true
+  default_formula 'mysql'
 
   satisfy { which 'mysql_config' }
 
diff --git a/Library/Formula/shocco.rb b/Library/Formula/shocco.rb
index bd6959517..c30df7b24 100644
--- a/Library/Formula/shocco.rb
+++ b/Library/Formula/shocco.rb
@@ -10,6 +10,7 @@ end
 
 class MarkdownProvider < Requirement
   fatal true
+  default_formula 'markdown'
 
   satisfy { which 'markdown' }
 
diff --git a/Library/Formula/signing-party.rb b/Library/Formula/signing-party.rb
index dfa02fae5..ecbb8c6b7 100644
--- a/Library/Formula/signing-party.rb
+++ b/Library/Formula/signing-party.rb
@@ -2,6 +2,7 @@ require 'formula'
 
 class GnupgInstalled < Requirement
   fatal true
+  default_formula 'gnupg2'
 
   satisfy { which('gpg') || which('gpg2') }
 
diff --git a/Library/Homebrew/requirement.rb b/Library/Homebrew/requirement.rb
index c20273895..4cae1f246 100644
--- a/Library/Homebrew/requirement.rb
+++ b/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
diff --git a/Library/Homebrew/requirements.rb b/Library/Homebrew/requirements.rb
index 29e61b696..bdbe71ee0 100644
--- a/Library/Homebrew/requirements.rb
+++ b/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') }
 
diff --git a/Library/Homebrew/requirements/mpi_dependency.rb b/Library/Homebrew/requirements/mpi_dependency.rb
index b51efc303..e83f7d80c 100644
--- a/Library/Homebrew/requirements/mpi_dependency.rb
+++ b/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