The accessor pair can't be provided from different sources

When one of the accessor pair is added through a mixin or metaclass, Groovy behaves as if the other one did not exist.

Explanation kindly provided by Paul: "MetaClassImpl finds a MetaBeanProperty containing just the getter. When only a getter is found there is an incorrect assumption that a setter doesn't exist in the underlying class."

===
class Foo {
void setVal(v)

{ println "setVal: $v" }

}
@Category(Foo) class Getter {
def getVal()

{ println "getVal" }
}
class Test {
static def main(av) { Foo.mixin Getter def f = new Foo() f.setVal('foo') f.val = null // => groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: val for class: Foo }
}
===

or

===
class Foo {
void setVal(v) { println "setVal: $v" }
}
class Test {
static def main(av) {
Foo.metaClass.getVal={-> println "getval" }
def f = new Foo()
f.setVal('foo')
f.val = null // => groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: val for class: Foo
}
}
===

or

===
class Foo {
def getVal() { println "getVal" }

}
class Test {
static def main(av) {
Foo.metaClass.setVal=

{v-> }

def f = new Foo()
f.getVal()
f.val
}
}
===

etc.