Why does Kotlin compiler let's this trap compile? ...

# stdlib

[SecretX]

Why does Kotlin compiler let's this trap compile? I introduced an error in production today on my company because of this, I thought the compiler or the IDE would catch something like this, but apparently not.

ConcurrentHashMap<Int, String>().getOrPut(1) { null } // compiles just fine, but it'll throw in runtime
mutableMapOf<Int, String>().getOrPut(1) { null } // compiler error: Null can not be a value of a non-null type String

[SecretX]

I just found out that other people already had this issue before, long before actually.

https://youtrack.jetbrains.com/issue/KT-39382

Why is that not fixed yet? I'm very surprised by that.

[Big Chungus]

Because it's java class and java has no null safety guarantees?

[SecretX]

But getOrPut is a Kotlin extension function of stdlib, not a method of ConcurrentMap interface, so that makes no sense.

Here is its signature.

public inline fun <K, V> java.util.concurrent.ConcurrentMap<K, V>.getOrPut(key: K, defaultValue: () -> V): V { /* compiled code */ }

[Paul Griffith]

It's just a bug; sometimes bugs (if niche enough) don't get fixed promptly
https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/jvm/src/kotlin/collections/MapsJVM.kt#L71
You could look into contributing a fix?

[Richard Gomez]

What is actually the difference between the signature of those two that would allow the former to compile? Is it the difference in implementation that leads to different compilation determinations?

inline fun <K, V> ConcurrentMap<K, V>.getOrPut(key: K, defaultValue: () -> V): V
inline fun <K, V> MutableMap<K, V>.getOrPut(key: K, defaultValue: () -> V): V

https://github.com/JetBrains/kotlin/blob/1e378ed136e19d3c2d0026a8382fe5d2d24404a9/libraries/stdlib/jvm/src/kotlin/collections/MapsJVM.kt#L71
https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/collections/Maps.kt#L356

[Richard Gomez]

Nevermind, it seems like V from ConcurrentMap implicitly Any! whereas it's Any for MutableMap.

[Richard Gomez]

I opened a PR to fix this: https://github.com/jetbrains/kotlin/pull/4689. It may be naive and not work for cases where people actually want Any?, but I wanted to start the ball rolling.