Circular reference error when defining Record type

TypeScript Version: 4.0.2 (and 4.1.0-dev.20201015)

Search Terms: circularly, Record

Code

type T1 = { [key: string]: T1 }
type T2 = Record<string, T2>

Expected behavior: No compilation errors

Actual behavior: While type T1 compiles properly, type T2 reports Type alias 'T2' circularly references itself. even though they are inherently the same type. This was actually found by autofixing a new @typescript-eslint rule consistent-indexed-object-style

Playground Link: https://www.typescriptlang.org/play?ts=4.0.2#code/C4TwDgpgBAKgjFAvFA3lA2gawiAXFAZ2ACcBLAOwHMBdfeKAXwFgAoUSWAJiSgCUIAxgHtiAEwA8RMlQA0XAHxA

Related Issues: None found

[nscarcella]

@RyanCavanaugh not sure what’s under the hood of Record, but it does feel a bit inconsistent that some generics seem to work fine with recursions while others don’t:

type A<T> = T | Array<A<T>>  // This line works fine
type R = Record<string, R>      // this one doesn't

[RyanCavanaugh]

Certain circularities are allowed (e.g. T1) but other circularities aren’t, e.g.

type Identity<T> = T;
type T3 = Identity<T3>;

Generic instantiation is deferred, so at the point TS analyzes T2’s declaration, it can’t tell if it’s in the Record case (would be allowed) or the Identity case (would not be allowed). It’s only later in the process that we could tell that this is actually OK, but if it wasn’t OK, it’s “too late” to go back and start complaining about it.