When creating a mapped type of an indexed type it extends too liberal #32484

type constr<Source, Tgt> = {[K in keyof Source]: string } &Pick<Tgt, Exclude<keyof Tgt, keyof Source>> 


type s = constr<{}, {
  [key: string]: {
      a: string;
  };
}>
declare const q: s
q["asd"].a.substr(1) //see: a is a string
q["asd"].b  //and of course b does not exist (this line will have an error)

//but I expect the following line to error as well
const d: {[key: string]: {a: string, b: string}} = q
//and verify to be "no" (instead it is "yes")
type verify = s extends {[key: string]: {a: string, b: string}} ? "yes" : "no"

// the error _is_ shown if you construct a (seemingly identical) type by hand
type sManual =  {} & Pick<{
    [key: string]: {
        a: string;
    };
}, string | number>
declare const qManual: sManual
// see: error here
const dManual: {[key: string]: {a: string, b: string}} = qManual

//the error is also shown if I replace the indexed type with an actual property
type s2 = constr<{}, {
  asd: {
      a: string;
  };
}>
declare const q2: s2
q2.asd.a.substr(1) //this is the same
q2.asd.b //this also


//But this is now an error (as it should be)
const d2: {asd: {a: string, b: string}} = q2

//and this is "no" (as it should be)
type verify2 = s2 extends {asd: {a: string, b: string}} ? "yes" : "no"