Allows the usage of `:head` dependencies even when the most recent published…

Allows the usage of `:head` dependencies even when the most recent published version was a pre-release.

Closes https://github.com/CocoaPods/CocoaPods/issues/3212.

CHANGELOG.md

@@ -12,10 +12,15 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   [Boris Bügling](https://github.com/neonichu)
 
 * Fixes warnings, when the aggregate target doesn't contain any pod target, which is build,
-  because `PODS_FRAMEWORK_BUILD_PATH` was added to `FRAMEWORK_SEARCH_PATHS`, but never created.
+  because `PODS_FRAMEWORK_BUILD_PATH` was added to `FRAMEWORK_SEARCH_PATHS`, but never created.  
   [Marius Rackwitz](https://github.com/mrackwitz)
   [#3217](https://github.com/CocoaPods/CocoaPods/issues/3217)
 
+* Allows the usage of `:head` dependencies even when the most recent published
+  version was a pre-release.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3212](https://github.com/CocoaPods/CocoaPods/issues/3212)
+
 
 ## 0.36.0.rc.1
 

lib/cocoapods/resolver.rb

@@ -180,7 +180,7 @@ module Pod
         end
       requirement_satisfied && !(
         spec.version.prerelease? &&
-        existing_vertices.flat_map(&:requirements).none? { |r| r.prerelease? || r.external_source }
+        existing_vertices.flat_map(&:requirements).none? { |r| r.prerelease? || r.external_source || r.head? }
       )
     end
 

spec/unit/resolver_spec.rb

@@ -287,6 +287,25 @@ module Pod
         ]
       end
 
+      it 'allows pre-release spec versions when a requirement has a ' \
+         'HEAD source' do
+        @podfile = Podfile.new do
+          platform :ios
+          pod 'MainSpec', :head
+        end
+        spec = Spec.new do |s|
+          s.name         = 'MainSpec'
+          s.version      = '1.2.3-pre'
+          s.platform     = :ios
+        end
+        resolver = Resolver.new(config.sandbox, @podfile, empty_graph, SourcesManager.all)
+        resolver.expects(:find_cached_set).returns(Specification::Set::Head.new(spec))
+        specs = resolver.resolve.values.flatten.map(&:to_s).sort
+        specs.should == [
+          'MainSpec (HEAD based on 1.2.3-pre)',
+        ]
+      end
+
       it "marks a specification's version to be a HEAD version" do
         podfile = Podfile.new do
           platform :ios