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

[toinehartman]

Describe the bug

The following Rascal definition is ambiguous:

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