Stumbled across a fun bug today: <https://youtrack.jetbrains.com/issue/KT-60370/data-objects-cannot-...

# getting-started

[Ruckus]

Stumbled across a fun bug today: https://youtrack.jetbrains.com/issue/KT-60370/data-objects-cannot-have-a-function-named-copy
Is the copy function supposed to be reserved somehow for data objects? There's nothing about it in the 1.9.0 release notes, and the 1.8.20 release note say

Because a data object declaration is intended to be used as a singleton object, no copy() function is generated.

[Chris Lee]

Examples in the ticket show a class not a data object?

[Francesc]

when you create a data class, the compiler generates copy methods with default arguments. If you then define your own copy method, it won't compile because the method is already defined. It's not reserved, it's just that the method is automatically generated and your own one clashes with it

[Chris Lee]

the ticket refers to the new data object construct that doesn’t support copy as objects are singletons.

[Francesc]

yes, a data object is just a singleton so a copy does't make sense.

[Chris Lee]

correct. so if there is no copy method generated, why does Kotlin prevent providing one named that? (regardless of what that method does or whether its appropriate)

[Ruckus]

Sorry, that was a copy/paste error on my part. Those should all be `data object`s, not `class`es. Fixed in the ticket.

[Francesc]

it's interesting that the error is the same as for data classes, but the decompiled bytecode does not have a copy method

[Chris Lee]

exactly.

[Chris Lee]

likely somewhere in the compiler theres something shared between data objects and data classes that gets this wrong.

[Ruckus]

On a data class, you can define a copy method with a different signature than the constructor without a problem. You will only see the conflicting overload error when they match. The compiler failing entirely in the second example seems unique to data objects.

[Chris Lee]

yea, it shouldn’t be possible for syntactically valid Kotlin to crash the compiler. bug somewhere.