Unable to add numeric types that are not simply 'number'

[DanielRosenwasser]

I'm trying to rewrite the signature of sum from core.ts in our compiler (see #16823), but I'm getting this error:

export function sum<T extends Record<K, number>, K extends string>(array: T[], prop: K): number {
 let result = 0;
 for (const v of array) {
 result += v[prop];
 // ~~~~~~~~~~~~~~~~~
 // Operator '+=' cannot be applied to types 'number' and 'T[K]'.
 }
 return result;
}

Note that I can't declare sum as

export function sum<K extends string>(array: Record<K, number>[], prop: K): number

because then TypeScript would infer all of the properties in each element for K. So I do need another type argument T for the indirection.

For a simpler repro:

function foo<T extends number>(x: T) {
 return 0 + x;
 // ~~~~~
 // Operator '+' cannot be applied to types '0' and 'T'.

}

Related is #15645.

Related: #15501

[sandersn]

Fix is up at #17870

[sandersn]

Note that #17404 fixed the simpler repro; #17870 fixes the original issue with a type parameter indexed by another type parameter.