How does the Groovy in operator work?

Question

The Groovy \"in\" operator seems to mean different things in different cases. Sometimes x in y means y.contains(x) and sometimes it seems to call

Answer 1

in is the "Membership operator".

From the documentation for Groovy 3 (emphasis mine):

8.6. Membership operator

The membership operator (in) is equivalent to calling the isCase method. In the context of a List, it is equivalent to calling contains, like in the following example:

def list = ['Grace','Rob','Emmy']
assert ('Emmy' in list)           # (1)    

(1) equivalent to calling list.contains('Emmy') or list.isCase('Emmy')

So, Groovy always calls isCase, which in case of a List maps to contains.