Type alias circular reference does not error and causes implicit any

[skovy]

TypeScript Version:

• typescript@next
• typescript@2.9.2

Search Terms:

• circular reference
• circular reference returntype
• Type alias circularly references itself returntype
• Type alias circularly references itself implicit any

Code

type A = ReturnType<A>;
//  ^^^ Type alias 'A' circularly references itself.

interface Input {
  a: number;
  b: number;
}

type R = ReturnType<typeof mul>;
function mul(input: Input): R {
  return input.a * input.b;
}
// ^^^ Return value of `mul` is now `any` because of the circular reference

Expected behavior:
Expected error: Type alias 'R' circularly references itself. (identical to A: Type alias 'A' circularly references itself.)

Actual behavior:
Only error: Type alias 'A' circularly references itself. (nothing about R or mul and the circular type reference)

Reasoning:
In this example, the mul function is simple but we use this for functions that can return complex objects (deeply nested) that already have explicit types. We then pass type R to other generics to provide type safety. This pattern works well, until we accidentally circular reference type R as in the above example. It's easy to miss and continue using because type R is now any so we (almost) never run into type errors as a result of this.

Playground Link

Potentially Related Issues:

• JS type argument defaulting should give implicit 'any' error #25609

Reproduces without the parameter:

type R = ReturnType<typeof f>;
function f(): R { return 0; }