support coercion from optional type to string within placeholder (#156)

RELEASE_NOTES.md

@@ -3,6 +3,7 @@
 ## in develop
 
 * Allows automatic coersion of `read_*` return values from `String` to other primitive types
+* Allows automatic coercion of optional-typed expressions to String within placeholders
 * Fixes handling of relative imports that are not in a subdirectory of the main WDL's parent directory
 
 ## 0.14.0 (2021-05-27)

src/main/scala/wdlTools/types/Unification.scala

@@ -122,6 +122,10 @@ case class Unification(regime: TypeCheckingRegime, logger: Logger = Logger.get)
         // Other coercions are not generally allowed, but are either allowed
         // in specific contexts or are used often and so allowed under less
         // strict regimes
+        case (T_String, T_Optional(r)) if ctx.inPlaceholder =>
+          // Within a placeholder, an optional value can be coerced to a String -
+          // None values result in the empty string
+          inner(innerTo, r, Enum.max(minPriority, Priority.ContextAllowed))
         case (T_String, T_Boolean | T_Int | T_Float) if ctx.inPlaceholder =>
           Some(Enum.max(minPriority, Priority.ContextAllowed))
         case (T_String, T_Boolean | T_Int | T_Float) if regime <= Lenient =>