Type 'any' is not assignable to type 'never' with boolean in interface in TS 3.5

[danvk]

TypeScript Version: 3.5.1

Search Terms:

• Type 'any' is not assignable to type 'never'

Code

interface T {
  str: string;
  bool: boolean;
}

const initState: T = {
  str: 'date',
  bool: true,
};

   let k: keyof T;
   initState[k] = 'test' as any;
// ~~~~~~~~~~~~ Type 'any' is not assignable to type 'never'.ts(2322)

This seems to happen specifically with boolean properties. If I get rid of the boolean-valued property from the interface it works fine. If I have a larger interface and select non-boolean properties, it also works fine. But boolean and another type produces this error.

This error happens with TS 3.5 but not 3.4 (see playground link).

Expected behavior:

No error.

Actual behavior:

Type 'any' is not assignable to type 'never'

Playground Link: link

Related Issues:

[RyanCavanaugh]

This is a documented breaking change; see "Fixes to unsound writes to indexed access types" in the release notes https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/

[danvk]

Thanks @RyanCavanaugh. But I don't follow why this is unsound. k is keyof T so it should be a valid index. I can use it to read from initState just fine:

const v = initState[k];  // ok

The right-hand side has an any type, so it should be fine to assign to any property in T.

[danvk]

Just to elaborate on why this seems odd... changing boolean to number in the interface makes the error go away, which doesn't seem like it should affect the soundness of the assignment.

interface T {
  str: string;
  num: number;
}

const initState: T = {
  str: 'date',
  num: 2,
};

let k: keyof T;
initState[k] = 'test' as any;  // ok

[danvk]

I guess it comes down to this, which also doesn't make a ton of sense to me (also in earlier TS versions):

type T1 = number & string;  // number & string
type T2 = string & boolean;  // never

[RyanCavanaugh]

The break is desired but the inconsistency between boolean (which is secretly a union) and number is not.

[KeithHenry]

This has caused a very large number of errors for us with insufficient guidance of how to fix (from the release notes)...

Most instances of this error represent potential errors in the relevant code. If you are convinced that you are not dealing with an error, you can use a type assertion instead.

OK, so we have large numbers of a pattern that looks like (from the OP):

initState[k] = v; // where k: keyof T and v: T[k]

And these all fail because we have lots of boolean properties, and so TypeScript thinks the type of initState[k] is never (surely that's wrong?)

Is the solution:

• Remove boolean from all interfaces? What should be used instead? Is boolean deprecated now?
• Some way to explicitly handle when the value of property k is boolean that then gates it out of the initState[k] check?
• Switch all instances that use keyof to just deal with any instead and lose type checking? Something like...

        (initState as any)[k] = v;

[danvk]

@KeithHenry the gist of the breaking change was that you never had type safety in the assignment to begin with. If you want the old behavior, you can put an as any around the initState[k]:

interface T {
  str: string;
  bool: boolean;
}

const initState: T = {
  str: 'date',
  bool: true,
};

declare let k: keyof T;
initState[k] = 'test';  // Type '"test"' is not assignable to type 'never'.
initState[k] = 'test' as any;  // Type 'any' is not assignable to type 'never'.
(initState[k] as any) = 'test';  // ok

[danvk]

Alternatively, you can construct a narrower type than keyof T which consists of only those keys whose values are assignable to boolean. You can do this quite generally with a conditional type:

interface T {
  str: string;
  bool: boolean;
  bool2: boolean;
  d: Date;
}

type KeysOfType<T, U> = { [k in keyof T]: T[k] extends U ? k : never }[keyof T];

const initState: T = {
  str: 'date',
  bool: true,
  bool2: false,
  d: new Date(),
};

declare let k: KeysOfType<T, boolean>;  // let k: "bool" | "bool2"
initState[k] = 'test';  // Type '"test"' is not assignable to type 'boolean'.
initState[k] = true;  // ok

[KeithHenry]

@danvk Thanks for the clarification, that helps.

Our current solution is to replace all initState[k] = 'test' with (initState as any)[k] = 'test', it sounds like that's the correct practice - we can then use KeysOfType where we need to check.