[Resolver] Allow pre-release spec versions when a requirement has an external source

Closes https://github.com/CocoaPods/CocoaPods/issues/2768.

CHANGELOG.md

@@ -4,6 +4,16 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
 
 To install release candidates run `[sudo] gem install cocoapods --pre`
 
+## Master
+
+##### Bug Fixes
+
+* Allows pre-release spec versions when a requirement has an external source
+  specified.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#2768](https://github.com/CocoaPods/CocoaPods/issues/2768)
+
+
 ## 0.35.0.rc1
 
 ##### Highlighted Enhancements That Need Testing

lib/cocoapods/resolver.rb

@@ -181,7 +181,10 @@ module Pod
         else
           requirement.requirement.satisfied_by? spec.version
         end
-      requirement_satisfied && !(spec.version.prerelease? && existing_vertices.flat_map(&:requirements).none?(&:prerelease?))
+      requirement_satisfied && !(
+        spec.version.prerelease? &&
+        existing_vertices.flat_map(&:requirements).none? { |r| r.prerelease? || r.external_source }
+      )
     end
 
     # Sort dependencies so that the ones that are easiest to resolve are first.

spec/unit/resolver_spec.rb

@@ -257,6 +257,25 @@ module Pod
         )
       end
 
+      it 'allows pre-release spec versions when a requirement has an ' \
+         'external source' do
+        @podfile = Podfile.new do
+          platform :ios
+          pod 'MainSpec', :git => 'GIT-URL'
+        end
+        spec = Spec.new do |s|
+          s.name         = 'MainSpec'
+          s.version      = '1.2.3-pre'
+          s.platform     = :ios
+        end
+        config.sandbox.expects(:specification).with('MainSpec').returns(spec)
+        resolver = Resolver.new(config.sandbox, @podfile, empty_graph, SourcesManager.all)
+        specs = resolver.resolve.values.flatten.map(&:to_s).sort
+        specs.should == [
+          'MainSpec (1.2.3-pre)'
+        ]
+      end
+
       it "marks a specification's version to be a HEAD version" do
         podfile = Podfile.new do
           platform :ios