We had a case where someone wrote a Stratego rule that has a wildcard in a build pattern:

  foo:
    bar -> _

The IDE produces an error, but this error is hidden behind the red cross on line 1. Specifically, the error does not show up on the wildcard term itself such that the error easily goes unnoticed. If you build the project the error does not show up. However, subsequent loading the language fails because the .ctree cannot be loaded (with an org.spoofax.interpreter.core.InterpreterException: Unknown op 'SRule/1').

tl;dr: Buliding a wildcard is an error but the build succeeds nevertheless resulting in errors that are confusing to Stratego/Spoofax newcomers.

Submitted by Martijn on 21 April 2018 at 14:27