let c = { [X in keyof { "foo" }]: { foo: "bar" } extends { X } ? true : false }["foo"];
let d = { foo: "bar" } extends { "foo" } ? true : false;
c and d should both return true, but surprisingly, c is evaluated as false.
Can anyone help shed light on this discrepancy? Perhaps it's a TypeScript issue?
Please note that this was tested with TypeScript version 4.4.4.
After filing a report on the issue with TypeScript's official repository, I received a response from RyanCavanaugh that provided a solution to the problem.
type x = { bar: "foo" } extends { "bar" } ? true : false; type y = { [Z in keyof { "bar" }]: { bar: "foo" } extends { Z } ? true : false }["bar"];The specific type being resolved here (highlighted for clarity) is
{ bar: "foo" } extends { Z: any } ? true : falseIn this context, K represents a property name and not a reference to the key type. An alternative representation could be written as
type x = { bar: "foo" } extends { "bar" } ? true : false; type y = { [Z in keyof { "bar" }]: { bar: "foo" } extends { QQQ } ? true : false }["bar"];As shown, the result is clearly
falseHere is an example where the result is
true:type y = { [Z in keyof { "bar": any }]: ({ bar: "foo" } extends Record<Z, unknown> ? true : false) }["bar"];
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