Discriminated union does not work with string literal types

[alex-kinokon]

TypeScript Version: 3.8.3

Search Terms:

Code

const $type: "@type" = "@type"
type A = { ["@type"]: "A", a: boolean }
type B = { ["@type"]: "B", b: boolean }
type C = A | B

declare const c: C;
switch (c[$type]) {
    case "A":
        c.a; // <-- Property 'a' does not exist on type 'B'.(2339)
}

switch (c["@type"]) {
    case "A":
        c.a;
}

Expected behavior:
No error.

Actual behavior:
Property 'a' does not exist on type 'B'.(2339)

Playground Link:
Playground

Related Issues:
Can’t find any so far.

[RyanCavanaugh]

@sandersn design limitation?

[sandersn]

Performance limitation; it's too expensive to put a control flow node on every element access -- creating it uses too much space and walking over it uses too much time. The decision to create a flow node is made syntactically, just after parsing, so there's no type information to guide it.

We have made similarly expensive expansions to control flow analysis before, like the nodes we add to support assert narrowing. But this feature isn't as useful, I think.

[sandersn]

Discussion is here, including a link back to the original bug: #31478