Inherit pod defined values for SWIFT_ACTIVE_COMPILATION_CONDITIONS in debug mode.

lib/cocoapods/generator/xcconfig/pod_xcconfig.rb

@@ -54,6 +54,7 @@ module Pod
             'PODS_TARGET_SRCROOT' => target.pod_target_srcroot,
             'PRODUCT_BUNDLE_IDENTIFIER' => 'org.cocoapods.${PRODUCT_NAME:rfc1034identifier}',
             'SKIP_INSTALL' => 'YES',
+            'SWIFT_ACTIVE_COMPILATION_CONDITIONS' => '$(inherited) ',
             # 'USE_HEADERMAP' => 'NO'
           }
 

lib/cocoapods/installer/xcode/pods_project_generator/pod_target_installer.rb

@@ -59,6 +59,8 @@ module Pod
             settings['CODE_SIGN_IDENTITY[sdk=iphoneos*]'] = ''
             settings['CODE_SIGN_IDENTITY[sdk=watchos*]'] = ''
 
+            settings['SWIFT_ACTIVE_COMPILATION_CONDITIONS'] = '$(inherited) '
+
             if target.swift_version
               settings['SWIFT_VERSION'] = target.swift_version
             end

lib/cocoapods/project.rb

@@ -254,6 +254,11 @@ module Pod
         end
       end
       settings['GCC_PREPROCESSOR_DEFINITIONS'] = definitions
+
+      if type == :debug
+        settings['SWIFT_ACTIVE_COMPILATION_CONDITIONS'] = 'DEBUG'
+      end
+
       build_configuration
     end