Can any1 help me understand why the built in type inference kotlinlang #codereview

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Title

m

myanmarking

01/28/2022, 7:45 PM

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

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()

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

out K

there.

i can even do:

temp - context

. It must be some compiler bug i guess

j

Javier

01/28/2022, 7:56 PM

K = String, 1 = Int

m

myanmarking

01/28/2022, 7:56 PM

?

j

Joffrey

01/28/2022, 8:07 PM

You're allowed to do this because

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.

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.

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