[Resolver] Make the version conflict message explain when deployment targets conflict

CHANGELOG.md

@@ -22,6 +22,11 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   [Samuel Giddins](https://github.com/segiddins)
   [#3764](https://github.com/CocoaPods/CocoaPods/issues/3764)
 
+* The resolver error message when a conflict occurred due to platform deployment
+  target mismatches will now explain that.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3926](https://github.com/CocoaPods/CocoaPods/issues/3926)
+
 
 ## 0.38.2
 

lib/cocoapods/resolver.rb

@@ -370,22 +370,30 @@ module Pod
     # @param  [Molinillo::ResolverError] error
     #
     def handle_resolver_error(error)
+      message = error.message
       case error
       when Molinillo::VersionConflict
         error.conflicts.each do |name, conflict|
           lockfile_reqs = conflict.requirements[name_for_locking_dependency_source]
           if lockfile_reqs && lockfile_reqs.last && lockfile_reqs.last.prerelease? && !conflict.existing
-            raise Informative, 'Due to the previous naïve CocoaPods resolver, ' \
+            message = 'Due to the previous naïve CocoaPods resolver, ' \
               "you were using a pre-release version of `#{name}`, " \
               'without explicitly asking for a pre-release version, which now leads to a conflict. ' \
               'Please decide to either use that pre-release version by adding the ' \
               'version requirement to your Podfile ' \
               "(e.g. `pod '#{name}', '#{lockfile_reqs.map(&:requirement).join("', '")}'`) " \
               "or revert to a stable version by running `pod update #{name}`."
+          elsif !conflict.existing
+            conflict.requirements.values.flatten.each do |r|
+              unless search_for(r).empty?
+                message << "\n\nSpecs satisfying the `#{r}` dependency were found, " \
+                  'but they required a higher minimum deployment target.'
+              end
+            end
           end
         end
       end
-      raise Informative, error.message
+      raise Informative, message
     end
 
     # Returns whether the given spec is platform-compatible with the dependency

spec/unit/resolver_spec.rb

@@ -191,6 +191,16 @@ module Pod
         ]
       end
 
+      it 'raises an informative error when version conflicts are caused by platform incompatibilities' do
+        @podfile = Podfile.new do
+          platform :osx, '10.7'
+          pod 'AFNetworking', '2.0.0' # requires 10.8
+        end
+        @resolver.stubs(:podfile).returns(@podfile)
+        message = should.raise(Informative) { @resolver.resolve }.message
+        message.should.match /required a higher minimum deployment target/
+      end
+
       it 'raises if unable to find a specification' do
         Specification.any_instance.stubs(:all_dependencies).returns([Dependency.new('Windows')])
         message = should.raise Informative do