Use polymorphism to simplify stdlib compatibility check

Library/Homebrew/build.rb

@@ -172,7 +172,7 @@ class Build
           # This currently only tracks a single C++ stdlib per dep,
           # though it's possible for different libs/executables in
           # a given formula to link to different ones.
-          stdlib_in_use = CxxStdlib.new(stdlibs.first, ENV.compiler)
+          stdlib_in_use = CxxStdlib.create(stdlibs.first, ENV.compiler)
           begin
             stdlib_in_use.check_dependencies(f, deps)
           rescue IncompatibleCxxStdlibs => e

Library/Homebrew/cxxstdlib.rb

@@ -1,37 +1,29 @@
 require "compilers"
 
 class CxxStdlib
-  attr_reader :type, :compiler
+  include CompilerConstants
 
-  def initialize(type, compiler)
+  def self.create(type, compiler)
     if type && ![:libstdcxx, :libcxx].include?(type)
       raise ArgumentError, "Invalid C++ stdlib type: #{type}"
     end
+    klass = GNU_GCC_REGEXP === compiler.to_s ? GnuStdlib : AppleStdlib
+    klass.new(type, compiler)
+  end
 
+  attr_reader :type, :compiler
+
+  def initialize(type, compiler)
     @type = type
     @compiler = compiler.to_sym
   end
 
-  def apple_compiler?
-    not compiler.to_s =~ CompilerConstants::GNU_GCC_REGEXP
-  end
-
-  def compatible_with?(other)
   # If either package doesn't use C++, all is well
-    return true if type.nil? || other.type.nil?
-
   # libstdc++ and libc++ aren't ever intercompatible
-    return false unless type == other.type
-
   # libstdc++ is compatible across Apple compilers, but
   # not between Apple and GNU compilers, or between GNU compiler versions
-    return false if apple_compiler? && !other.apple_compiler?
-    if compiler.to_s =~ CompilerConstants::GNU_GCC_REGEXP
-      return false unless other.compiler.to_s =~ CompilerConstants::GNU_GCC_REGEXP
-      return false unless compiler.to_s[4..6] == other.compiler.to_s[4..6]
-    end
-
-    true
+  def compatible_with?(other)
+    (type.nil? || other.type.nil?) || type == other.type
   end
 
   def check_dependencies(formula, deps)
@@ -53,4 +45,26 @@ class CxxStdlib
   def type_string
     type.to_s.gsub(/cxx$/, 'c++')
   end
+
+  class AppleStdlib < CxxStdlib
+    def apple_compiler?
+      true
+    end
+
+    def compatible_with?(other)
+      super && other.apple_compiler?
+    end
+  end
+
+  class GnuStdlib < CxxStdlib
+    def apple_compiler?
+      false
+    end
+
+    def compatible_with?(other)
+      super &&
+        !other.apple_compiler? &&
+        compiler.to_s[4..6] == other.compiler.to_s[4..6]
+    end
+  end
 end

Library/Homebrew/formula_installer.rb

@@ -159,7 +159,7 @@ class FormulaInstaller
         @poured_bottle = true
 
         stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs
-        stdlib_in_use = CxxStdlib.new(stdlibs.first, MacOS.default_compiler)
+        stdlib_in_use = CxxStdlib.create(stdlibs.first, MacOS.default_compiler)
         begin
           stdlib_in_use.check_dependencies(f, f.recursive_dependencies)
         rescue IncompatibleCxxStdlibs => e

Library/Homebrew/tab.rb

@@ -119,7 +119,7 @@ class Tab < OpenStruct
     # Older tabs won't have these values, so provide sensible defaults
     lib = stdlib.to_sym if stdlib
     cc = compiler || MacOS.default_compiler
-    CxxStdlib.new(lib, cc.to_sym)
+    CxxStdlib.create(lib, cc.to_sym)
   end
 
   def to_json

Library/Homebrew/test/test_stdlib.rb

@@ -5,14 +5,14 @@ require 'cxxstdlib'
 
 class CxxStdlibTests < Homebrew::TestCase
   def setup
-    @clang = CxxStdlib.new(:libstdcxx, :clang)
-    @gcc   = CxxStdlib.new(:libstdcxx, :gcc)
-    @llvm  = CxxStdlib.new(:libstdcxx, :llvm)
-    @gcc4  = CxxStdlib.new(:libstdcxx, :gcc_4_0)
-    @gcc48 = CxxStdlib.new(:libstdcxx, 'gcc-4.8')
-    @gcc49 = CxxStdlib.new(:libstdcxx, 'gcc-4.9')
-    @lcxx  = CxxStdlib.new(:libcxx, :clang)
-    @purec = CxxStdlib.new(nil, :clang)
+    @clang = CxxStdlib.create(:libstdcxx, :clang)
+    @gcc   = CxxStdlib.create(:libstdcxx, :gcc)
+    @llvm  = CxxStdlib.create(:libstdcxx, :llvm)
+    @gcc4  = CxxStdlib.create(:libstdcxx, :gcc_4_0)
+    @gcc48 = CxxStdlib.create(:libstdcxx, 'gcc-4.8')
+    @gcc49 = CxxStdlib.create(:libstdcxx, 'gcc-4.9')
+    @lcxx  = CxxStdlib.create(:libcxx, :clang)
+    @purec = CxxStdlib.create(nil, :clang)
   end
 
   def test_apple_libstdcxx_intercompatibility