Ambiguity: function definition with `when` and brackets around condition

**Describe the bug**

The following Rascal definition is ambiguous:
```rascal
list[int] a(int i) = [i] when (p.def is \lex);
```

**Desktop (please complete the following information):**
 - Context: Eclipse plugin, Commandline REPL
 - Rascal Version: `0.42.0`
