There is some room for using a union in the extends clause of a generic definition. While you cannot use discriminator, you can compare against an array. The following object allows a type and an array of the same type. You can narrow down to any of the two types using instanceOf and manipulate the parameter value:

interface ObjectWithAge {
 kind: "ObjectWithAge";
 age: number;
}

function funct2<T extends ObjectWithAge | ObjectWithAge[]>(p: T): T {
 if (p instanceof Array) {
   return p[0];
 }
 return p;
}

Trying to extend two different objects with a discriminator does not work at the moment.