[XCConfigurationList] Ignore nil values in `common_resolved_build_set…

…ting`

See CocoaPods/CocoaPods#1462

CHANGELOG.md

@@ -2,8 +2,16 @@
 
 ###### Enhancements
 
-* [XCConfigurationList] Added default value for `default_configuration_name`
-  attribute.  
+* [AbstractTarget] The `#sdk` method now raises if the value is not the same
+  across all the build configurations.  
+  [Fabio Pelosin](https://github.com/irrationalfab)
+
+* [XCConfigurationList] `common_resolved_build_setting` will now ignore nil
+  values. This is an heuristic which might not closely match Xcode behaviour.
+  This is done because some information, like the SDK, is usually considered at
+  the target level but it might actually differ in the build configurations.
+  For example nothing prevents a target to build with the iOS sdk in one
+  configuration and with the OS X in another.  
   [Fabio Pelosin](https://github.com/irrationalfab)
 
 
@@ -12,7 +20,8 @@
 ###### Breaking
 
 * [AbstractTarget] The `#sdk` method now raises if the value is not the same
-  across all the build configurations.  
+  across all the build configurations. This has been done to prevent clients
+  from accidentally using arbitrary values.  
   [Fabio Pelosin](https://github.com/irrationalfab)
 
 ###### Enhancements

lib/xcodeproj/project/object/native_target.rb

@@ -67,14 +67,19 @@ def resolved_build_setting(key)
         #
         # @raise  If the build setting has multiple values.
         #
+        # @note   As it is common not to have a setting with no value for
+        #         custom build configurations nil keys are not considered to
+        #         determine if the setting is unique. This is an heuristic
+        #         which might not closely match Xcode behaviour.
+        #
         # @return [String] The value of the build setting.
         #
         def common_resolved_build_setting(key)
-          values = resolved_build_setting(key).values.uniq
-          if values.count == 1
+          values = resolved_build_setting(key).values.compact.uniq
+          if values.count <= 1
             values.first
           else
-            raise "Build setting `#{key}` has multiple values: `#{resolved_build_setting(key)}`"
+            raise "[Xcodeproj] Consistency issue: build setting `#{key}` has multiple values: `#{resolved_build_setting(key)}`"
           end
         end
 
@@ -95,7 +100,9 @@ def platform_name
         # @return [String] the version of the SDK.
         #
         def sdk_version
-          sdk.scan(/[0-9.]+/).first
+          if sdk
+            sdk.scan(/[0-9.]+/).first
+          end
         end
 
         # @return [String] the deployment target of the target according to its