When TypeScript encounters difficulty understanding that (T & Record<K, U>)[K] is equal to or a subtype of U, I tend to expand T & Record<K, U> to Record<K, U> through reassignment. This is because TypeScript does recognize that the simpler Record<K, U>[K] can be assigned to U. For example:
function foo<K extends `a${string}`>(k: K) {
const a = {} as { b: 1 } & Record<K, string>
const _a: Record<K, string> = a; // widen a
const r = _a[k];
r.toLowerCase();
}
In this scenario, _a is essentially just a, but its type has been expanded from
{b: 1} & Record<K, string>
to
Record<K, string>. Once you have this,
_a[k] can then be assigned to
string.
It's important to note that TypeScript isn't always sound and may not cover all scenarios, so generic type manipulations like these may succeed or fail unexpectedly. The practice of widening T & Record<K, U> to Record<K, U> is typically safe, but there are deficiencies in TS's type system that can disrupt equivalences in certain cases. If all else fails and you believe your approach is secure, resorting to a type assertion like const r = a[k] as string is acceptable.
Click here for Playground link to code