Currently the type coercion table does not list a supported type coercion from File to String, only String to File.

There are also conflicting examples in the spec regarding a File to String type coercion:

• coercion_fail.wdl which explicitly says File does not coerce to String in a call to the contains function and thus is an error.
• change_extension_task.wdl which explicitly shows a call to sub with a File argument, despite the type of sub being [String, String, String] -> String.

In an attempt to be faithful to the spec, sprocket is implementing full static analysis (and soon evaluation) of WDL and does not allow a coercion from File to String as it isn't in the coercion table; this is producing errors in existing WDL source that is accepted by Cromwell.

Is a coercion from File (and also Directory) to String intended to be accepted in the current WDL spec? If so, we should include it in the coercion table and correct the coercion_fail.wdl example.

If not, we should fix the change_extension_task.wdl example.