type Example1<T extends string, U extends string> = keyof (Record<T, any> & Record<U, any>)
type Result1 = Example1<'x', 'y'>
// Type of Result1 is never but it should be 'x' | 'y'


/**  The additional examples below are just for context */

type Result2 = keyof (Record<'x', any> & Record<'y', any>) 
// Type of Result2 is 'x' | 'y' as expected

type Example3<T extends string> = keyof (Record<T, any>)
type Result3 = Example3<'x' | 'y'>
// Type of Result3 is 'x' | 'y' as expected

type Example4<T extends string, U extends string> = (Record<T, any> & Record<U, any>)
type Result4 = keyof Example4<'x', 'y'>
// Type of Result4 is 'x' | 'y' as expected

type Example5<T, U> = keyof (T & U)
type Result5 = Example5<Record<'x', any>, Record<'y', any>>
// Type of Result5 is 'x' | 'y' as expected