Lint rule for definite assignment assertions with immediate initialization

[DanielRosenwasser]

var x!: number = 2;

class Class {
  a!: number = 10
}

Seems like a weird construct. In #20117 we said we should write a lint rule for this.

[ajafff]

@DanielRosenwasser I just tried typescript@next and I get an error from typescript for both definite assignment assertions.

A definite assignment assertion '!' is not permitted in this context.

That means there is no need for a lint rule for declarations with initializer.

---

I'm currently only linting for assertions that are unnecessary because the type is an undefined union:

let foo!: string | undefined; // unnecessary assertion

---

class Foo {
    prop!: string; // unnecessary assertion
    constructor() {
        this.prop = 1;
    }
}

To detect if a definite assignment assertion is unnecessary because the property is initialized in the constructor (see above), we'd need a new API (similar to #20221 but specialized for properties)

[mhegazy]

@ajafff u are right about the new API. I think this can be lumped with #20221, asking if the property is definitely assigned at the end of the constructor.

[mhegazy]

Marking as Working as Intended since the examples in the OP are already reported as errors.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.