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Repeated identical conditional return type triggers TS2719 ("Two different types with this name exist") under tsgo #3488
Description
Steps to reproduce
tsconfig.json:
{
"compilerOptions": {
"strict": true,
"target": "ES2022",
"module": "ESNext",
"moduleResolution": "Bundler",
"noEmit": true
}
}index.ts:
export function cast<T>(value: T): {
as: <K extends T>() => null extends T ? K | null : undefined extends T ? K | undefined : K;
} {
return {
as: <K extends T>(): null extends T ? K | null : undefined extends T ? K | undefined : K => {
return value as K;
},
};
}Tested with @typescript/native-preview@beta (Version 7.0.0-dev.20260421.2) and [email protected].
Workaround: extract the conditional into a single named alias so both annotation sites reference the same type symbol.
type PreserveNullability<T, K extends T> = null extends T
? K | null
: undefined extends T
? K | undefined
: K;
export function cast<T>(value: T): {
as: <K extends T>() => PreserveNullability<T, K>;
} {
return {
as: <K extends T>(): PreserveNullability<T, K> => {
return value as PreserveNullability<T, K>;
},
};
}Behavior with [email protected]
Compiles cleanly (exit 0, no diagnostics).
Behavior with tsgo
index.ts(5,5): error TS2719: Type '<K extends T>() => null extends T ? K | null : undefined extends T ? K | undefined : K' is not assignable to type '<K extends T>() => null extends T ? K | null : undefined extends T ? K | undefined : K'. Two different types with this name exist, but they are unrelated.
Type 'null extends T ? K | (T & undefined) | (T & null) : undefined extends T ? K | (T & undefined) | (T & null) : K | (T & undefined) | (T & null)' is not assignable to type 'null extends T ? K | (T & null) : undefined extends T ? K | (T & undefined) : K'.
Type 'K | (undefined extends T ? K | (T & undefined) | (T & null) : K | (T & undefined) | (T & null)) | (T & undefined) | (T & null)' is not assignable to type 'null extends T ? K | (T & null) : undefined extends T ? K | (T & undefined) : K'.
Type 'undefined extends T ? K | (T & undefined) | (T & null) : K | (T & undefined) | (T & null)' is not assignable to type 'null extends T ? K | (T & null) : undefined extends T ? K | (T & undefined) : K'.
Type 'K | (T & undefined) | (T & null)' is not assignable to type 'null extends T ? K | (T & null) : undefined extends T ? K | (T & undefined) : K'.
Type 'T & undefined' is not assignable to type 'null extends T ? K | (T & null) : undefined extends T ? K | (T & undefined) : K'.
Type 'K | (T & undefined) | (T & null)' is not assignable to type 'K | (T & null)'.
Type 'T & undefined' is not assignable to type 'K | (T & null)'.
Type 'T & undefined' is not assignable to type 'T & null'.
Type 'T & undefined' is not assignable to type 'null'.
The two types in the diagnostic are written out identically. Note also that in tsgo's elaboration the type has been re-distributed (K | (T & undefined) | (T & null)) compared with what the user wrote (K | null or K | undefined), suggesting the conditional is being reduced differently in the two positions before identity is checked.
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