Nullish coalescing should always include the type of the right operand

$ ./node_modules/.bin/tsc --version
Version 3.7.2

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Code

A toy example would be something like this.

let foo: string = "";
foo = "bar" ?? 123;

This one is obviously fine since "bar" is always truthy.

However, this becomes a little bit problematic when you consider the idiom of having Record objects and checking their truthiness before using them.

const elts = ["foo", "bar", "spam", "spam", "foo", "eggs"];
const counts: Record<string, number>;
for (const elt of elts) {
  // This really **should** raise an error.
  counts[elt] = counts[elt] ?? "zero";
  counts[elt] += 1;
}

Expected behavior:
An error should be raised.

Actual behavior:
Curiously, an error is not raised in strict mode but is raised in un-strict mode.

$ ./node_modules/.bin/tsc ./foo.ts 
foo.ts:5:3 - error TS2322: Type 'number | "zero"' is not assignable to type 'number'.
  Type '"zero"' is not assignable to type 'number'.

5   counts[elt] = counts[elt] ?? "zero";
    ~~~~~~~~~~~


Found 1 error.

$ ./node_modules/.bin/tsc --strict ./foo.ts
# No error, exits 0 and emits JS.

Playground Link:
Playground Link

Toggling the strictNullChecks config option will show the issue.

Related Issues:

Reproduisez les exemples avec TypeScript 3.7.2 en utilisant à la fois la commande par défaut du compilateur et --strict, puis comparez les diagnostics pour l’affectation avec coalescence nulle. Le travail est terminé lorsque l’expression signalée inclut systématiquement le type de l’opérande de droite et déclenche l’erreur attendue avec la configuration indiquée.