Distributive type on generic has surprising results

[ceymard]

🔎 Search Terms

remove generic string type from union containing it with string literals / filter generic string out of union containing literal types

🕗 Version & Regression Information

5.7

⏯ Playground Link

No response

💻 Code

type LiteralString<T> = string extends T ? never : T;

type Test1 = LiteralString<string>; // never
type Test2 = LiteralString<"literal">; // "literal"
type Test3 = LiteralString<string | "literal"> // shoud be "literal"

🙁 Actual behavior

Test3 ends up being never

🙂 Expected behavior

Test3 should be "literal"

Additional information about the issue

No response

[jcalz]

• That's not a distributive conditional type. In string extends T, the type string is not a generic type parameter, so it is just a plain conditional type, it does not distribute across unions.

• Even if it were distributive, this could never work; the type string | "literal" is eagerly reduced to string, so there is no way for Test3 to behave differently from Test1, no matter how LiteralString<T> were implemented.

This is not a bug in TypeScript.

[ceymard]

Is there a way to prevent that eagerness to keep the literal types ?

[jcalz]

See #29729; you could try things like #29729 (comment) (e.g., (string & {}) | "literal") but that could have weird side effects. Or are other things you can do, like wrapping your types in something else (e.g., [string] | ["literal"]) but again, that would involve some kind of refactoring. In any case this isn't a bug in TS and this is not a general support forum so I'll disengage. Good luck!