Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the rule's documentation

https://typescript-eslint.io/rules/no-unnecessary-condition/

Description

Report when checking type guards, where the type of the variable is already exactly equal to the type guard return type.

Fail

function isBool(x: unknown): x is boolean {
    return typeof x === 'boolean';
}

declare const b: boolean;

if (isBool(b)) { // this is unnecessary, since b is already boolean.
}

Pass

declare const maybeBoolean: boolean | string;

if (isBool(maybeBoolean)) { // necessary - might not a boolean.
}

declare const t: true;

if (isBool(t)) { // debatable - t is a subtype of boolean, not exactly equal. this has some funny effects in the type system.
    console.log(t); // t gets widened to boolean here, even though it was typed as true.
} else {
    console.error(t); // t also widened to boolean here.
}

Additional Info

#9076 is in a similar domain, checking unnecessary assertion functions.