ENV.with_build_environment

Library/Homebrew/extend/ENV.rb

@@ -438,6 +438,14 @@ class << ENV
     self['PATH'] = paths.unshift(*self['PATH'].split(":")).uniq.join(":")
   end
 
+  def with_build_environment
+    old_env = to_hash
+    setup_build_environment
+    yield
+  ensure
+    replace(old_env)
+  end
+
   def fortran
     fc_flag_vars = %w{FCFLAGS FFLAGS}
 

Library/Homebrew/requirements.rb

@@ -125,10 +125,10 @@ class MPIDependency < Requirement
 
   def satisfied?
     # we have to assure the ENV is (almost) as during the build
-    orig_PATH = ENV['PATH']
     require 'superenv'
-    ENV.setup_build_environment
+    ENV.with_build_environment do
       ENV.userpaths!
+
       @lang_list.each do |lang|
         case lang
         when :cc, :cxx, :f90, :f77
@@ -138,10 +138,7 @@ class MPIDependency < Requirement
           @unknown_langs << lang.to_s
         end
       end
-
-    # Restore the original paths
-    ENV['PATH'] = orig_PATH
-
+    end
     @unknown_langs.empty? and @non_functional.empty?
   end
 

Library/Homebrew/test/test_ENV.rb

@@ -26,4 +26,13 @@ class EnvironmentTests < Test::Unit::TestCase
     assert_nil ENV['LD']
     assert_equal ENV['OBJC'], ENV['CC']
   end
+
+  def test_with_build_environment
+    before = ENV.to_hash
+    ENV.with_build_environment do
+      ENV['foo'] = 'bar'
+    end
+    assert_nil ENV['foo']
+    assert_equal before, ENV.to_hash
+  end
 end