Can any1 help me understand why the built-in type ...

# codereview

[myanmarking]

Can any1 help me understand why the built-in type inference for map is not working

[myanmarking]

val map: Map<String, Data> = ..
val temp = map - 1

public operator fun <K, V> Map<out K, V>.minus(key: K): Map<K, V> =
this.toMutableMap().apply { minusAssign(key) }.optimizeReadOnlyMap()

[myanmarking]

why am i allowed to do that (temp variable) without the compiler complaining. Clearly there is out K there.

[myanmarking]

i can even do: temp - context. It must be some compiler bug i guess

[Javier]

K = String, 1 = Int

[myanmarking]

?

[Joffrey]

You're allowed to do this because K is then inferred to Any for temp. Try to add an explicit type for the temp variable, you'll see that trying Map<String, Data> shouldn't work.

[Joffrey]

Clearly there is out K there.

This is exactly why you can do this. A Map<String, Data> conforms to Map<out Any, Data> , so you can call the minus function with K=Any on this Map<String, Data> without issues.