After submitting a support ticket, it was suggested I report this issue here.

I have a workflow that runs iOS tests, using caches for both Carthage and Cocoapods dependencies. For the Carthage dependencies, the cache key expression looks like this:

${{ runner.os }}-carthage-${{ hashFiles('**/Cartfile.resolved') }}

Recently a workflow failed with this error:

Module compiled with Swift 5.1.3 cannot be imported by the Swift 5.2.2 compiler

because the cache was used but the Swift version on the MacOS runner had been updated.

I believe that the solution to this is to include the Swift version in the cache key expression, just like runner.os and the hash of Cartfile.resolved. Running something like swift --version will output the version and I was thinking of using this in a step's output so that it could be read as input in the cache key but I understand that cache key expressions are evaluated before the workflow is executed.

So it would be great to either get access to the Swift version in a similar fashion to runner.os or to be able to use step outputs as inputs in the cache key expression.