T[K] is not assignable to NonNullable<T>[K] even when T is nonnull (regression)

[falsandtru]

TypeScript Version: 3.2.0-dev.20180929

Search Terms:

Code

Revert the change of source code to see the error: falsandtru/clientchannel@14cc53a

Expected behavior:

pass

Actual behavior:

TypeScript error: src/layer/domain/indexeddb/service/channel.ts(48,15): Error TS2322: Type 'V[Exclude<keyof V, "__meta" | "__id" | "__key" | "__date" | "__event">] | undefined' is not assignable to type 'NonNullable<Partial<V>>[Exclude<keyof V, "__meta" | "__id" | "__key" | "__date" | "__event">]'.
  Type 'undefined' is not assignable to type 'NonNullable<Partial<V>>[Exclude<keyof V, "__meta" | "__id" | "__key" | "__date" | "__event">]'.

Playground Link:

Related Issues:

[ahejlsberg]

I'm not sure there is a regression here. The following is an error with 3.0 as well:

// Compile with --strictNullChecks
function foo<T, K extends keyof T>(x: T[K], y: NonNullable<T>[K]) {
  x = y;
  y = x;  // Error
}

I think the error is reasonable. After all, NonNullable<T> is a subtype of T.

[falsandtru]

Compiling passed probably since 2018-3-5.

https://github.com/falsandtru/clientchannel/blob/0611018e8b840abbcd7a9d49f08c9dc12daa847b/src/layer/domain/indexeddb/service/channel.ts
https://travis-ci.org/falsandtru/clientchannel/builds

Generally, subtype isn't replaceable by base type. But if T is originally nonnull, T and NonNullable<T> are the same type (base type is replaceable by subtype in this case). And if T is nullable, property access should be error. What do you think?

[falsandtru]

@ahejlsberg Can you take a look at the requested info?

[falsandtru]

@ahejlsberg