Closed
Description
openedon Mar 7, 2020
TypeScript Version: Nightly
Search Terms:
generic, conditional type, cannot be assigned, generic return value
Expected behavior:
A value of type X is a valid return value for a function returning Always<X>, since the latter is a generic conditional type that always evaluates to X.
Actual behavior:
Does not compile. Error message:
Type 'X' is not assignable to type 'Always<X>'. (2322)
Related Issues:
#31751 seems to be more about never type semantics.
Code
You can substitute any type for any in the definition below and it will fail regardless.
export type Always<X> = X extends any ? X : X
const f = <X>(): Always<X> => /*error*/ null as unknown as X /*error*/Output
const f = () => null;
Compiler Options
{
"compilerOptions": {
"noImplicitAny": true,
"strictNullChecks": true,
"strictFunctionTypes": true,
"strictPropertyInitialization": true,
"strictBindCallApply": true,
"noImplicitThis": true,
"noImplicitReturns": true,
"useDefineForClassFields": false,
"alwaysStrict": true,
"allowUnreachableCode": false,
"allowUnusedLabels": false,
"downlevelIteration": false,
"noEmitHelpers": false,
"noLib": false,
"noStrictGenericChecks": false,
"noUnusedLocals": false,
"noUnusedParameters": false,
"esModuleInterop": true,
"preserveConstEnums": false,
"removeComments": false,
"skipLibCheck": false,
"checkJs": false,
"allowJs": false,
"declaration": true,
"experimentalDecorators": false,
"emitDecoratorMetadata": false,
"target": "ES2017",
"module": "ESNext"
}
}Playground Link: Provided
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