Diagnostic about "left-hand side of a 'for...in' statement" type is out of date

[mattmccutchen]

TypeScript Version: 2.1.5

Now that the left side of a 'for...in' statement is apparently allowed to be keyof the right side, the diagnostic for when the left side has the wrong type needs to be updated to account for that possibility.

(Aside: Is it intentional that the left side won't be inferred to have the keyof type because of the unsoundness described in #12314, but it is allowed to be explicitly declared as that type?)

Code

interface Foo {
  x: number;
  y: string;
}

function scan(f: Foo) {
  let k: number;
  for (k in f) {  // Error: "The left-hand side of a 'for...in' statement must be of type 'string' or 'any'."
    console.log(k);
  }
  let k2: keyof Foo;
  for (k2 in f) {  // No error
    console.log(k2);
  }
}

Expected behavior:
Something like "The index type of the right side of a 'for...in' statement must be assignable to the left side" perhaps, with details on the failure of assignability?

Actual behavior:
"The left-hand side of a 'for...in' statement must be of type 'string' or 'any'."

[mhegazy]

keyof T is string, so why is this error message wrong?

[mattmccutchen]

keyof T is string, so why is this error message wrong?

I assume you meant keyof Foo. I don't think so: when I mouse over k2 in Visual Studio Code, it shows the type as "x" | "y". In any case, if I change the annotation to "x" | "y", there is no error, so the error message stating that the type must be string or any is definitely wrong.

[mhegazy]

"x" | "y" is a subtype of string. so saying that k needs to be "x" | "y" | string is more or less the same as saying it should be string.

[mhegazy]

also the constraint is not really keyof T where T is the type of the right-hand-side, the constraint is string..

i.e. this is allowed:

  let k: "x" | "y" | "z";
  for (k in f) { 
    console.log(k);
  }

The reason is at run-time , f can have more properties than "x" and "y".

[mattmccutchen]

Then how do you explain that let k: "x" is an error, but let k: "x" | "y" | Date is allowed? The rule as implemented appears to be that keyof T must be assignable to the type of the left side, and the diagnostic should reflect that.

[RyanCavanaugh]

Seems to not be a problem in practice but we'd take a PR if someone wrote a good improvement on the existing message