CxxStdlib: allow for nil stdlibs

A nil stdlib value represents non-C++ code.

Library/Homebrew/cxxstdlib.rb

@@ -2,11 +2,11 @@ class CxxStdlib
   attr_accessor :type, :compiler
 
   def initialize(type, compiler)
-    if ![:libstdcxx, :libcxx].include? type
+    if type && ![:libstdcxx, :libcxx].include?(type)
       raise ArgumentError, "Invalid C++ stdlib type: #{type}"
     end
 
-    @type     = type.to_sym
+    @type     = type.to_sym if type
     @compiler = compiler.to_sym
   end
 
@@ -15,6 +15,9 @@ class CxxStdlib
   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
 

Library/Homebrew/test/test_stdlib.rb

@@ -13,6 +13,7 @@ class CxxStdlibTests < Test::Unit::TestCase
     @gcc48 = CxxStdlib.new(:libstdcxx, 'gcc-4.8')
     @gcc49 = CxxStdlib.new(:libstdcxx, 'gcc-4.9')
     @lcxx  = CxxStdlib.new(:libcxx, :clang)
+    @purec = CxxStdlib.new(nil, :clang)
   end
 
   def test_apple_libstdcxx_intercompatibility
@@ -59,4 +60,8 @@ class CxxStdlibTests < Test::Unit::TestCase
     assert_equal stdlib.compiler, :clang
     assert_equal stdlib.type, :libstdcxx
   end
+
+  def test_compatibility_for_non_cxx_software
+    assert @purec.compatible_with?(@clang)
+  end
 end