Expando Metaclass behaviour dependant on whether a class is user defined or not?

Question

While experimenting with ExpandMetaClass I ran into this behaviour:

class A {}

A.metaClass.foo = { 0 }
A.metaClass.foo = { 1 }
println (new A().foo())
A.metaClass.foo = { 2 }
println (new A().foo())

Collection.metaClass.foo = { 0 }
Collection.metaClass.foo = { 1 }
println ([].foo())
Collection.metaClass.foo = { 2 }
println ([].foo())

Which prints

1
2
1
1

Which doesn't seem to be consistent. What's the rationale behind this behavior?

Answer 1

Sounds like a bug. You should fill a JIRA.

It works per instance:

class A {}

A.metaClass.foo = { 0 }
A.metaClass.foo = { 1 }
assert (new A().foo()) == 1
A.metaClass.foo = { 2 }
assert (new A().foo()) == 2

Collection.metaClass.foo = { 0 }
Collection.metaClass.foo = { 1 }
assert ([].foo()) == 1

def j = []
j.getMetaClass().foo = { 2 }
assert (j.foo()) == 2