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Control flow analysis for dependent parameters doesn't work for methodsΒ #48160
Description
Bug Report
π Search Terms
control flow analysis, dependent parameters, methods, #47190, #48110
π Version & Regression Information
This behavior has been present since #47190 was merged, so TS4.6.
β― Playground Link
Playground link with relevant code
π» Code
interface Foo {
method(...args: ['a', number] | ['b', string]): void
};
const methodSyntax: Foo = {
method(kind, payload) {
if (kind === 'a') {
payload.toFixed(); // error, Property 'toFixed' does not exist on type 'string | number'
}
if (kind === 'b') {
payload.toUpperCase(); // error, Property 'toUpperCase' does not exist on type 'string | number'
}
}
}π Actual behavior
Checking kind inside the body of method() does not narrow payload from string | number.
π Expected behavior
Given #47190, I'd expect kind === 'a' to narrow payload to number, and kind === 'b' to narrow payload to string, which does happen when method is a function expression:
const funcExpr: Foo = {
method: function (kind, payload) {
if (kind === 'a') {
payload.toFixed() // okay
}
if (kind === 'b') {
payload.toUpperCase(); // okay
}
}
}And when method is an arrow function:
const arrowFunc: Foo = {
method: (kind, payload) => {
if (kind === 'a') {
payload.toFixed(); // okay
}
if (kind === 'b') {
payload.toUpperCase(); // okay
}
}
}And by #46266 when destructured from a rest argument:
const manualDestructuring: Foo = {
method(...args) {
const [kind, payload] = args;
if (kind === 'a') {
payload.toFixed(); // okay
}
if (kind === 'b') {
payload.toUpperCase(); // okay
}
}
} I don't know whether this is really a bug or just a follow-up feature request to add to #46658 / #47190. For context: I mentioned this in #47190, and Mateusz BurzyΕski (@Andarist) has written a PR for it at #48110.
Activity
RyanCavanaugh commentedon Mar 8, 2022
Seems like an oversight