Optional chaining breaking function inference.

[ShanonJackson]

TypeScript Version: 3.7.2

Search Terms:
optional chaining
Code

interface Maybe<A> { };
export declare function Maybe<A>(value: A | null | undefined): Maybe<A>;
declare const ctx: {req?: {headers?: {cookie?: string}}}
const testopt = ctx.req?.headers?.cookie
const teststring = "" as string | undefined;
const test = Maybe(testopt) // Maybe<string | undefined> (wrong)
const test1 = Maybe(teststring ) // Maybe<string>  (correct)

Expected behavior:
const test should be of type Maybe<string> like const test1 because this is the behavior of inference on a function with this signature.

Actual behavior:
const test is of type Maybe<string | undefined;

[fatcerberus]

This is quite bizarre. It would be understandable if there were some bug where it inferred different types based on the form of the expression passed to Maybe(). But here they're both assigned to a variable first, which should equalize things--yet it doesn't.

Since the type of cookie is already string | undefined, could it be that the optional chain is actually yielding a type of string | undefined | undefined and this is not being properly normalized... only to become observable later when used as an inference source for A.

[MartinJohns]

Playground link:
https://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgLJwJ4CMIB4CCAfMgN7IC+A3ALABQEAHgA4D2UYyAJhAgDZxQUMAK4gEYYCxBpMOAoQAUANzi9hEAFzJ8yAD7IQw3rz3JR3GKAicAlFvTY8RGrW58BKBFIDOHcQy0SQQBHAH5AgAsIOG4ob3DSLxYAa2AIBN8oUABzcjy6LxBfZEhfFiYOAF5kfwA6ENDaqJjoeNqk1IgCnw5SsEyc5GqAImHkOG9kAZBs03MISxBrF0LivqGZRwU+8rAbZAB6A825adn9ecXrYgUAdygpbJtuot6IXwBGDYccbff+sBZGb7I4nPBnYjIBReKCCcTPWhAA

[ajafff]

Should be fixed by #34588 which is already available in typescript@next and will be in the next patch release.

[RyanCavanaugh]

Y'all know you can try "Nightly" in the Playground, right?

Duplicate #34579