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Discriminated union does not work with string literal types #37650
Description
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.
Activity
RyanCavanaugh commentedon Apr 1, 2020
Nathan Shively-Sanders (@sandersn) design limitation?
sandersn commentedon Apr 1, 2020
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
assertnarrowing. But this feature isn't as useful, I think.sandersn commentedon Apr 1, 2020
Discussion is here, including a link back to the original bug: #31478