Separate formula conflicts from requirements

Closes Homebrew/homebrew#20357.

Library/Homebrew/cmd/audit.rb

@@ -174,11 +174,11 @@ class FormulaAuditor
   end
 
   def audit_conflicts
-    f.conflicts.each do |req|
+    f.conflicts.each do |c|
       begin
-        Formula.factory req.formula
+        Formula.factory(c.name)
       rescue FormulaUnavailableError
-        problem "Can't find conflicting formula \"#{req.formula}\"."
+        problem "Can't find conflicting formula #{c.name.inspect}."
       end
     end
   end

Library/Homebrew/cmd/info.rb

@@ -102,7 +102,7 @@ module Homebrew extend self
       puts
     end
 
-    conflicts = f.conflicts.map(&:formula).sort!
+    conflicts = f.conflicts.map(&:name).sort!
     puts "Conflicts with: #{conflicts*', '}" unless conflicts.empty?
 
     if f.rack.directory?

Library/Homebrew/exceptions.rb

@@ -115,6 +115,38 @@ class UnsatisfiedRequirements < Homebrew::InstallationError
   end
 end
 
+class FormulaConflictError < Homebrew::InstallationError
+  attr_reader :f, :conflicts
+
+  def initialize(f, conflicts)
+    @f = f
+    @conflicts = conflicts
+    super f, message
+  end
+
+  def conflict_message(conflict)
+    message = []
+    message << "  #{conflict.name}"
+    message << ": because #{conflict.reason}" if conflict.reason
+    message.join
+  end
+
+  def message
+    message = []
+    message << "Cannot install #{f.name} because conflicting formulae are installed.\n"
+    message.concat conflicts.map { |c| conflict_message(c) } << ""
+    message << <<-EOS.undent
+      Please `brew unlink #{conflicts.map(&:name)*' '}` before continuing.
+
+      Unlinking removes a formula's symlinks from #{HOMEBREW_PREFIX}. You can
+      link the formula again after the install finishes. You can --force this
+      install, but the build may fail or cause obscure side-effects in the
+      resulting software.
+      EOS
+    message.join("\n")
+  end
+end
+
 class BuildError < Homebrew::InstallationError
   attr_reader :exit_status, :command, :env
 

Library/Homebrew/formula.rb

@@ -491,7 +491,7 @@ class Formula
   end
 
   def conflicts
-    requirements.grep(ConflictRequirement)
+    self.class.conflicts
   end
 
   # Returns a list of Dependency objects in an installable order, which
@@ -764,8 +764,12 @@ class Formula
       @plist_manual = options[:manual]
     end
 
-    def conflicts_with formula, opts={}
-      dependencies.add ConflictRequirement.new(formula, name, opts)
+    def conflicts
+      @conflicts ||= []
+    end
+
+    def conflicts_with name, opts={}
+      conflicts << FormulaConflict.new(name, opts[:because])
     end
 
     def skip_clean *paths

Library/Homebrew/formula_installer.rb

@@ -78,6 +78,8 @@ class FormulaInstaller
       EOS
     end
 
+    check_conflicts
+
     unless ignore_deps
       perform_readline_hack
       check_requirements
@@ -121,6 +123,17 @@ class FormulaInstaller
     end
   end
 
+  def check_conflicts
+    return if ARGV.force?
+
+    conflicts = f.conflicts.reject do |c|
+      keg = Formula.factory(c.name).prefix
+      not keg.directory? && Keg.new(keg).linked?
+    end
+
+    raise FormulaConflictError.new(f, conflicts) unless conflicts.empty?
+  end
+
   def check_requirements
     unsatisfied = ARGV.filter_for_dependencies do
       f.recursive_requirements do |dependent, req|

Library/Homebrew/formula_support.rb

@@ -2,6 +2,8 @@ require 'download_strategy'
 require 'checksum'
 require 'version'
 
+FormulaConflict = Struct.new(:name, :reason)
+
 class SoftwareSpec
   attr_reader :checksum, :mirrors, :specs
   attr_reader :using # for auditing

Library/Homebrew/requirements.rb

@@ -1,5 +1,4 @@
 require 'requirement'
-require 'requirements/conflict_requirement'
 require 'requirements/language_module_dependency'
 require 'requirements/x11_dependency'
 require 'requirements/mpi_dependency'

Library/Homebrew/requirements/conflict_requirement.rb

@@ -1,33 +0,0 @@
-require 'requirement'
-
-# This requirement added by the `conflicts_with` DSL method.
-class ConflictRequirement < Requirement
-  attr_reader :formula
-
-  # The user can chose to force installation even in the face of conflicts.
-  fatal !ARGV.force?
-
-  def initialize formula, name, opts={}
-    @formula = formula
-    @name = name
-    @opts = opts
-    super([formula])
-  end
-
-  def message
-    message = "#{@name.downcase} cannot be installed alongside #{@formula}.\n"
-    message << "This is because #{@opts[:because]}\n" if @opts[:because]
-    message << <<-EOS.undent unless ARGV.force?
-      Please `brew unlink #{@formula}` before continuing. Unlinking removes
-      the formula's symlinks from #{HOMEBREW_PREFIX}. You can link the
-      formula again after the install finishes. You can --force this install
-      but the build may fail or cause obscure side-effects in the end-binary.
-    EOS
-    message
-  end
-
-  satisfy :build_env => false do
-    keg = Formula.factory(@formula).prefix
-    not keg.exist? && Keg.new(keg).linked?
-  end
-end