T[keyof T] types as constraints

[stevekane]

TypeScript Version: 2.2.0-dev.20161125

Code

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"

Expected behavior:
I would expect the value of a type indexing itself to in-fact be all the types of its values. This should be knowable at compile-time and especially so if the type is constrained as T | never

Actual behavior:
Compiler does not recognize that U[keyof U] is the same as T.

[mhegazy]

I am not not sure i understand the issue. so bear with me..
I do not see how the maps is related tot he boxed version. Mapof<T> forgets all the properties by vertue of the indexer. so it does not fit with the updateBoxed function. i would expect this to be typed as:

type Boxed<T> = {
 [P in keyof T]: { value: T[P] }
}
function updateBoxed<U> (b: Boxed<U>, u: U) {
 for (var k in u) {
 b[k].value = u[k]
 }
}

// usage

var obj = { a: 1, b: "string" };

var boxed = boxify(obj);

updateBoxed(boxed, { a: 2 });// OK
updateBoxed(boxed, { a: 2, b: "anotherString" });// OK
updateBoxed(boxed, { a: false });// Error

[ahejlsberg]

@stevekane Check out #12528 for an example of how to implement this.