Show some elaboration for property not existing in union type

[RyanCavanaugh]

I have some code like this, except there are about a dozen types instead of just three:

interface Foo {
  kind: "foo";
  name: string;
}
interface Bar {
  kind: "bar";
  name: string;
}
interface Qua {
  kind: "qua";
  length: string;
}

type FBQ = Foo | Bar | Qua;
let x: FBQ = <any>undefined;
console.log(x.name);

The error reported is

a.ts(16,15): error TS2339: Property 'name' does not exist on type 'FBQ'.

This is inactionable on a union type, especially when its consituents are hidden behind an alias. We should show something like

a.ts(16,15): error TS2339: Property 'name' does not exist on type 'FBQ'.
   Property 'name' does not exist on type 'Qua'

[yahiko00]

Yes please! More generally, there are some rooms to improvement in compiler error messages.

[RyanCavanaugh]

Accepting PRs / committed for 2.1. Show an elaboration for the first type lacking the property (for whatever definition of 'first' falls out of the implementation)

[ojsheikh]

Should there also be an elaboration when one of the constituent types has the property marked as private or protected and we are in a public context? Currently the code:

class Default {
    member: string;
}
class Private {
    private member: number;
}

var DP: Default | Private;
DP.member;

gives the error:

test.ts(9,4): error TS2339: Property 'member' does not exist on type 'Default | Private'.

This could perhaps be changed to:

test.ts(9,4): error TS2339: Property 'member' does not exist on type 'Default | Private'.
  Property 'member' is private and only accessible within class 'Private'.

I have adjusted checkPropertyAccessExpressionOrQualifiedName to show an elaboration when a subtype of a union lacks a property and will open a PR once I've finished adding tests. It should not be difficult to also show an elaboration when a subtype has the property marked under a stricter access qualifier.

[mhegazy]

thanks @oijazsh !