Capture success on failure if type is annotated or infered to `P[Unit]`

[fabianhjr]

scalac: v2.13.8
fastparse: v2.3.3

Observing this behaviour from repl:

@ import fastparse._ 
import fastparse._

@ import NoWhitespace._ 
import NoWhitespace._

@ def fail[T: P] = Fail.! 
defined function fail

@ val wat = "Shouldn't success" 
wat: String = "Shouldn't success"

@ parse(wat, fail(_)) 
res4: Parsed[String] = Parsed.Failure(Position 1:1, found "Shouldn't ")

@ def fail[T: P]: P[Unit] = Fail.! 
defined function fail

@ parse(wat, fail(_)) 
res6: Parsed[Unit] = Success(value = (), index = 0)