Why can't an index signature parameter type extend string or number?

I wrote this simple function:

function propertyMap<T extends string>(prop:T)  {      return function<U extends {[key:T]:V},V>(record:U):V      {          return record[prop];      };  };  

And the compiler is giving me the following errors:

error TS1023: An index signature parameter type must be 'string' or 'number'.    3  return function<U extends {[key:T]:V},V>(record:U):V                                  ~~~  error TS2536: Type 'T' cannot be used to index type 'U'.    5   return record[prop];                 ~~~~~~~~~~~~  

Why is this a limitation?

https://stackoverflow.com/questions/65557167/why-cant-an-index-signature-parameter-type-extend-string-or-number January 04, 2021 at 10:59AM