While experimenting with some code, I encountered an issue where the generic type of a property value wasn't being resolved correctly when changing from TValue to (t: TValue) => TValue. Instead of being recognized as the expected number, it was now seen as an unknown {} type.
An example without a function works as intended:
type ProtoOf<T> = Pick<T, keyof T>;
function decorate<TValue>(value: TValue) {
return <T extends { [KA in TKey]: TValue }, TKey extends keyof T>(
proto: ProtoOf<T> & { [P in TKey]: TValue },
propertyKey: TKey
) => {};
}
class Foo {
// No TS error
@decorate(1) bar: number = 1;
// TS Error
@decorate('') wrongBar: number = 1;
}
However, an example involving a function does not work as expected:
type ProtoOf<T> = Pick<T, keyof T>;
function decorate<TValue>(getValue: (t: TValue) => TValue) {
return <T extends { [KA in TKey]: TValue }, TKey extends keyof T>(
proto: ProtoOf<T> & { [P in TKey]: TValue },
propertyKey: TKey
) => {};
}
class Foo {
// TS Error
@decorate(v => v + 1) bar: number = 1;
// No TS error
@decorate(v => v + '') wrongBar: number = 1;
}
The goal was to have TValue equal to number in both examples, but that wasn't achieved in the second case involving a function. This discrepancy was unexpected.