Type inference for constructor parameters with default values

[mfursov]

Hi guys,

We have type inference for variable declarations but why don’t we have it for primary constructor parameters with default values?

fun f() {
    val f = 1 // SUCCESS
}

but

class A(
    val a = 1 // ERROR
)

[tieskedh]

Why can not type of function parameters with default value be omited?

The main reason is compilation performance. When we look at a method call, it helps a lot to have explicit parameter types which don’t need to be inferred. Inferring the parameter type from a string constant is trivial, but if you have fun foo(a = bar()) and bar() also has an inferred return type, understanding the actual parameter type of a becomes very expensive.

But there is an issue on Youtrack for what you want:

https://youtrack.jetbrains.com/issue/KT-19046

[mfursov]

Thanks! I missed that topic.