Do not alter swift_versions attribute when calculating Swift versions.

CHANGELOG.md

@@ -8,7 +8,9 @@
 
 ##### Bug Fixes
 
-* None.  
+* Do not alter `swift_versions` attribute when calculating Swift versions.  
+  [Dimitris Koutsogiorgas](https://github.com/dnkoutso)
+  [#XXX](https://github.com/CocoaPods/Core/pull/XXX)
 
 
 ## 1.8.0 (2019-09-23)

lib/cocoapods-core/specification/root_attribute_accessors.rb

@@ -47,7 +47,7 @@ def swift_version
         #
         def swift_versions
           @swift_versions ||= begin
-            swift_versions = Array(attributes_hash['swift_versions'])
+            swift_versions = Array(attributes_hash['swift_versions']).dup
             # Pre 1.7.0, the DSL was singularized as it supported only a single version of Swift. In 1.7.0 the DSL
             # is now pluralized always and a specification can support multiple versions of Swift. This ensures
             # we parse the old JSON serialized format and include it as part of the Swift versions supported.

spec/specification/json_spec.rb

@@ -374,6 +374,17 @@ module Pod
           hash['swift_version'].should == '1.0'
         end
 
+        it 'does not alter the attribute hash when loading swift versions' do
+          @spec.swift_versions = ['4.2', '5.0']
+          first_hash = @spec.to_hash
+          first_hash['swift_versions'].should == ['4.2', '5.0']
+          first_hash['swift_version'].should == '5.0'
+          spec_from_hash = Specification.from_hash(first_hash)
+          second_hash = spec_from_hash.to_hash
+          second_hash['swift_versions'].should == ['4.2', '5.0']
+          second_hash['swift_version'].should == '5.0'
+        end
+
         it 'reads swift version from a string' do
           hash = {
             'name' => 'BananaLib',