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Intrinsic string mapping type with type parameters is incorrectly resolved in conditional types #55847
Description
Activity
Workaround:
type A<S extends string> = Lowercase<S> extends "foo" ? 1 : 0 type B<S extends string> = Lowercase<S> extends `f${string}` ? 1 : 0
Not sure why you do the intersection with
string. That seems rather odd.Yes, I admit that its more natural to use type constraints in above example.
In fact, I found this issue while using
Lowercasecombined with mapped type.
But it turned out that mapped type was not the cause of problem and I decided to use more simple example in above description.Here is a more motivating example.
type CamelCase<S extends string> = Lowercase<S> extends `${infer S1}_${infer S2}` ? `${S1}${Capitalize<S2>}` : Lowercase<S> type RenameKeys<T> = { [K in keyof T as CamelCase<K&string>]: T[K] } type A = CamelCase<"foo_bar"> // ^? type A = "fooBar" type B = RenameKeys<{foo_bar: number}> // ^? type B = { foo_bar: number } // should be B = { fooBar: number }
I think it is natural to use
K&stringin this example because we want to consider only string keys.I think it is natural to use
K&stringin this example because we want to consider only string keys.Why not
keyof T & stringorExtract<keyof T, string>?Martin Johns (@MartinJohns) I think that makes the mapped type not homomorphic
Reacted by Martin Johns and Godai Takashina- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
Bug Report
🔎 Search Terms
lowercase, uppercase, intrinsic, conditional type
🕗 Version & Regression Information
⏯ Playground Link
https://www.typescriptlang.org/play?ts=5.1.6#code/C4TwDgpgBAggPAZQHxQLxQDIHsDuEBOAxgIYDOEiAZKcPgJYB2A5ihAB7AQMAmpUARADMsWflAD8UAIxQAXFAAMAKAD0KqFAB64paEhQAQohTpseImQoJqtRiyjtOPPgANBAEgDeNeswC+LhLScoqq6lo6SgA2EMBQbFLy8EIi-CbBag74+Fj40bHxAEzyRimi6TKZBDn4QA
💻 Code
🙁 Actual behavior
A<"foo"> = 0andB<"foo"> = 0.Actually, these conditional branches are resolved at the time of type alias definition.
Then, any instantiation of
AandBis0🙂 Expected behavior
Both
A<"foo">andB<"foo">should be1.Conditional types in A and B should not be resolved during type alias definition, but should be deferred until an instantiation.
Additional information about the issue
Related
#52102 is similar.
This issue is about incorrect "simplification" involving intrinsic string mapping types.
More info
I did some investigation in codebase.
It looks like that the cause of the problem is handling of string mapping type in
getGenericObjectFlagsorisPatternLiteralPlaceholderTypewhich was introduced in #52836 .https://github.dev/microsoft/TypeScript/blob/97147915ab667a52e31ac743843786cbc9049559/src/compiler/checker.ts#L17967