[Resolver] Fix multi-source resolution

CHANGELOG.md

@@ -18,6 +18,11 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   [Dimitris Koutsogiorgas](https://github.com/dnkoutso)
   [#7292](https://github.com/CocoaPods/CocoaPods/pull/7292)
 
+* Fix resolution when multiple sources provide the same pods, and there are 
+  (potential) dependencies between the sources.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#7031](https://github.com/CococaPods/CocoaPods/issues/7031)
+
 ## 1.4.0.rc.1 (2017-12-16)
 
 ##### Enhancements

lib/cocoapods/resolver.rb

@@ -351,8 +351,7 @@ module Pod
         all_specifications(installation_options.warn_for_multiple_pod_sources).
         select { |s| requirement.satisfied_by? s.version }.
         map { |s| s.subspec_by_name(dependency.name, false, true) }.
-        compact.
-        reverse
+        compact
     end
 
     # @return [Set] Loads or returns a previously initialized set for the Pod
@@ -508,7 +507,7 @@ module Pod
           end,
         )
       end
-      raise type, message
+      raise type.new(message).tap { |e| e.set_backtrace(error.backtrace) }
     end
 
     # Returns whether the given spec is platform-compatible with the dependency

lib/cocoapods/resolver/lazy_specification.rb

@@ -37,12 +37,12 @@ module Pod
         end
       end
 
+      # returns the highest versioned spec last
       def all_specifications(warn_for_multiple_pod_sources)
         @all_specifications ||= begin
           sources_by_version = {}
           versions_by_source.each do |source, versions|
             versions.each { |v| (sources_by_version[v] ||= []) << source }
-            sources_by_version
           end
 
           if warn_for_multiple_pod_sources
@@ -56,8 +56,11 @@ module Pod
             end
           end
 
-          versions_by_source.flat_map do |source, versions|
-            versions.map { |version| LazySpecification.new(name, version, source) }
+          # sort versions from high to low
+          sources_by_version.sort_by(&:first).flat_map do |version, sources|
+            # within each version, we want the prefered (first-specified) source
+            # to be the _last_ one
+            sources.reverse_each.map { |source| LazySpecification.new(name, version, source) }
           end
         end
       end