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Conditional type doesn't go to true or false branch. #48243
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Activity
Resolving of conditional types involving unbound generic type arguments is deferred. At that point the compiler doesn't know what
Action<P, T>resolves to, because the compiler doesn't know what typesPandTare.Martin Johns (@MartinJohns) thanks for clarification, but i still don't understand. why it doesn't know what result of conditional type if i'm telling it that T will extend Record<string, unknown>
Tcan still benever, asneveralso extendsRecord<string, unknown>. TypeScript simply does not resolve conditional types involving unbound generic types, even if theoretically in some specific corner cases it could.Martin Johns (@MartinJohns) playground but why this works ?
if i'm telling it that T will extend Record<string, unknown>
Conditional types are not necessarily linear (meaning that they have predictable behavior between
Tand an a subtype ofT), and figuring out whether or not they are requires reasoning in the form of "Does any type exist such that this type would behave in the other way?", which is not very tractable.Ryan Cavanaugh (@RyanCavanaugh) okey, but why [T] extends [never] and T extends never produce different results (you can see the example above) ?
Aliaksei Molchan (@alexeymolchan) https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types
T extends ...distributes over unions.[T] extends [...]does not. The former is thus more likely to be deferred than the latter, since it can't know whether to enable the distributive behavior or not until it has a concrete type to work with.Bruce Pascoe (@fatcerberus) as i see from playground where
T extends ...no defer occur and i can accessaction.meta. correct me please, if i'm wrong.Martin Johns (@MartinJohns) mapped type like this
type OptionsFlags<Type> = { [Property in keyof Type]: boolean; };will be deferred too ?How would the compiler know what the final type will look like without knowing what type
Tactually is?To be clear, generics in TS don't work like C++ templates. The compiler only looks at the body of a generic once and tries to determine if it will work for all possible combinations of type parameters, but these checks are necessarily conservative because it would be prohibitively expensive and/or complex to do otherwise.
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- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
Bug Report
🔎 Search Terms
generic conditional types
already checked https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs and #31751
🕗 Version & Regression Information
3.9.7 / 4.6.2
⏯ Playground Link
Playground link with relevant code
💻 Code
🙁 Actual behavior
properties from intersection are not accessible (conditional type doesn't resolve as true / false branch)
🙂 Expected behavior
conditional type should go to false branch. correct my please if i'm wrong
Thanks in advance.