T[keyof T] types as constraints #12509

type MapOf<T> { [ x: string ]: T | never } // values only of type T
type Box<T> { value: T }

// Here we show that the above type constrains valid types for values of MapOf<T> to T
const t: MapOf<number> = { foo: 1 } // compiles
const tbad: MapOf<number> = { foo: 'foo' } // does not compile

// assign every value of MapOf<T> to some T
function forMapsOf <T> ( m: MapOf<T>, t: T ) {
  for ( var k in m ) { 
    m[k] = t
  }
} // Compiles and works as expected

// now let's transform the definition of the above function slightly
function forMapsOf <T, M extends MapOf<T>> ( m: M, t: T ) {
  for ( var k in m ) { 
    m[k] = t
  }
} // DOES NOT COMPILE - Says "Type T is not assignable to type M[keyof M]

// The above is crucial for this function below which would take a map of 
// Boxed values and update them from a map of unboxed values
function updateBoxed 
<T, 
 U extends Only<T>, 
 B extends { [ K in keyof U ]: Box<U[K]> }> 
( b: B, u: U ) {
  for ( var k in u ) { 
    b[k].value = u[k]
  }
} // DOES NOT COMPILE - Says "Type U[keyof U] is not assignable to type T"