Output Swift targets when multiple versions of Swift are detected.

CHANGELOG.md

@@ -8,6 +8,10 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
 
 ##### Enhancements
 
+* Output Swift targets when multiple versions of Swift are detected.  
+  [Justin Martin](https://github.com/justinseanmartin) & [Dimitris Koutsogiorgas](https://github.com/dnkoutso)
+  [#6191](https://github.com/CocoaPods/CocoaPods/issues/6191)
+
 * [update] adding --sources to specify to only update pods from a repo  
   [Mark Schall](https://github.com/maschall)
   [#5809](https://github.com/CocoaPods/CocoaPods/pull/5809)

lib/cocoapods/installer/analyzer/target_inspector.rb

@@ -216,16 +216,34 @@ module Pod
         # @return [String] the targets Swift version or nil
         #
         def compute_swift_version_from_targets(targets)
-          versions = targets.flat_map do |target|
-            target.resolved_build_setting('SWIFT_VERSION').values
-          end.flatten.compact.uniq
-          case versions.count
+          versions_to_targets = targets.inject({}) do |memo, target|
+            versions = target.resolved_build_setting('SWIFT_VERSION').values
+            versions.each do |version|
+              memo[version] = [] if memo[version].nil?
+              memo[version] << target.name unless memo[version].include? target.name
+            end
+            memo
+          end
+
+          case versions_to_targets.count
           when 0
             nil
           when 1
-            versions.first
+            versions_to_targets.keys.first
           else
-            raise Informative, 'There may only be up to 1 unique SWIFT_VERSION per target.'
+            target_version_pairs = versions_to_targets.map do |version_names, target_names|
+              target_names.map { |target_name| [target_name, version_names] }
+            end
+
+            sorted_pairs = target_version_pairs.flat_map { |i| i }.sort_by do |target_name, version_name|
+              "#{target_name} #{version_name}"
+            end
+
+            formatted_output = sorted_pairs.map do |target, version_name|
+              "#{target}: Swift #{version_name}"
+            end.join("\n")
+
+            raise Informative, "There may only be up to 1 unique SWIFT_VERSION per target. Found target(s) with multiple Swift versions:\n#{formatted_output}"
           end
         end
       end

spec/unit/installer/analyzer/target_inspector_spec.rb

@@ -322,9 +322,32 @@ module Pod
         user_targets = [target]
 
         target_inspector = TargetInspector.new(target_definition, config.installation_root)
+
+        expected_versions_string = "Target: Swift 2.3\nTarget: Swift 3.0"
+
+        should.raise(Informative) do
+          target_inspector.send(:compute_swift_version_from_targets, user_targets)
+        end.message.should.include "There may only be up to 1 unique SWIFT_VERSION per target. Found target(s) with multiple Swift versions:\n#{expected_versions_string}"
+      end
+
+      it 'raises if the user defined SWIFT_VERSION contains multiple unique versions are defined on different targets' do
+        user_project = Xcodeproj::Project.new('path')
+        target = user_project.new_target(:application, 'Target', :ios)
+        target.build_configuration_list.set_setting('SWIFT_VERSION', '2.3')
+
+        target2 = user_project.new_target(:application, 'Target2', :ios)
+        target2.build_configuration_list.set_setting('SWIFT_VERSION', '3.0')
+
+        target_definition = Podfile::TargetDefinition.new(:default, nil)
+        user_targets = [target, target2]
+
+        target_inspector = TargetInspector.new(target_definition, config.installation_root)
+
+        expected_versions_string = "Target: Swift 2.3\nTarget2: Swift 3.0"
+
         should.raise(Informative) do
           target_inspector.send(:compute_swift_version_from_targets, user_targets)
-        end.message.should.include 'There may only be up to 1 unique SWIFT_VERSION per target.'
+        end.message.should.include "There may only be up to 1 unique SWIFT_VERSION per target. Found target(s) with multiple Swift versions:\n#{expected_versions_string}"
       end
 
       it 'returns the project-level SWIFT_VERSION if the target-level SWIFT_VERSION is not defined' do