"is assignable to the constraint of type but could be instantiated with a different subtype of constraint" error with assetion function

[mateja176]

TypeScript Version: 3.9.4

Search Terms:

is assignable to the constraint of type

Code

const isNil = <A extends any>(a: A) => a === undefined || a === null;

type DeepRequired<O extends {}> = {
  [key in keyof O]: O[key] extends {}
    ? DeepRequired<O[key]>
    : NonNullable<O[key]>;
};

export function assertNonNullable<O extends {}>(
  object: O,
): asserts object is DeepRequired<O> { // throws
  Object.entries(object).forEach(([key, value]) => {
    if (typeof value === 'object' && value !== null) {
      assertNonNullable(value);
    }
    if (isNil(value)) {
      throw new Error(`Key ${key} is required, but the provided value is nil`);
    }
  });
}

interface MaybeA {
  a?: number;
}

const a: MaybeA = { a: undefined };

assertNonNullable(a);

const b = a.a;

Expected behavior:

No error is thrown

Actual behavior:

The following error is thrown:

A type predicate's type must be assignable to its parameter's type.
  Type 'DeepRequired<O>' is not assignable to type 'O'.
    'DeepRequired<O>' is assignable to the constraint of type 'O', but 'O' could be instantiated with a different subtype of constraint '{}'.
      Type 'O[key] extends {} ? DeepRequired<O[key]> : NonNullable<O[key]>' is not assignable to type 'O[key]'.
        Type 'DeepRequired<O[key]> | NonNullable<O[key]>' is not assignable to type 'O[key]'.
          Type 'DeepRequired<O[key]>' is not assignable to type 'O[key]'.

Playground Link:

Link

Related Issues:

#29049

[fatcerberus]

It seems that the root of the issue is that, since O is an unbound type variable, TypeScript doesn't actually know DeepRequired<O> is related to O (it does know it's related to {} but this isn't enough, as the error message proves). For that the compiler would need to do some kind of higher-level reasoning (i.e. reasoning about the generic type alias itself rather than a specific instantiation of it), which it can't currently do.

My suspicion is therefore that this is probably a design limitation.