Extract to inner function drops narrowed type

[mjbvz]

Bug Report

🔎 Search Terms

• refactor
• refactoring

🕗 Version & Regression Information

4.8.0-dev.20220616, not a regression

💻 Code

function doThing(x: number | string) { 
    if (typeof x === 'string') {
        x.charCodeAt(0); // Run extract to inner function here
    }
}

🙁 Actual behavior

The resulting code is invalid:

function doThing(x: number | string) {
    if (typeof x === 'string') {
        newFunction();
    }

    function newFunction() {
        x.charCodeAt(0); // Error here: x is of type number | string
    }
}

🙂 Expected behavior

We either don't offer the refactoring in this case or we somehow pass the narrowed type to the function

[MartinJohns]

we somehow pass the narrowed type to the function

Related: #9998

[RyanCavanaugh]

I think the logic could be that for every closed-over identifier, if the narrowed type of that identifier is different from its declared type, it should be instead promoted to a parameter of the inner function:

function doThing(x: number | string) {
    if (typeof x === 'string') {
        newFunction(x);
    }

    function newFunction(x: string) {
        x.charCodeAt(0); // Error here: x is of type number | string
    }
}

Probably we'd want to rename x here as the local lexical shadowing is awful.

[bpasero]

Another example when you try to extract a method for the code that is in if (isFoo(foo)) {:

interface IFoo {
	id: string;
}

interface IBar {
	id: string;
}

function isFoo(obj: any): obj is IFoo {
	return typeof obj === 'string';
}

function migrate(foo: IBar | IFoo) {
	if (isFoo(foo)) {
		const bar = foo;
		console.log(bar);
	} else {
		console.log(foo);
	}
}