Repository: TypeScript
Issue: Discriminant-specific error message not shown when one constituent of a union has a union-typed discriminant
Labels: Suggestion,Experience Enhancement

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For npm: `typescript@next`
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**TypeScript Version:**  3.9.0-dev.20200319

<!-- Search terms you tried before logging this (so others can find this issue more easily) -->
**Search Terms:** discrimant tagged union assignable

**Code**

A simplified repro, based on real-world code:
```ts
interface A { x: number }
interface B { x: string }

interface U$A { kind: 'A', body: A }
interface U$B { kind: 'B', body: B }
interface U$null { kind: 'C' | 'D', body?: undefined }

type U = U$A | U$B | U$null;

declare function foo(u: U): void;

foo({kind: 'A', body: { x: 42 }});  // OK
foo({kind: 'B', body: { x: 42 }});  // Not OK
```

**Expected behavior:**
The second `x: 42` gets a squiggle and and the error `Type 'number' is not assignable to type 'string'.`

**Actual behavior:**
The whole argument to the second `foo` call gets a squiggle and the error:
>Argument of type '{ kind: "B"; body: { x: number; }; }' is not assignable to parameter of type 'U'. Type '{ kind: "B"; body: { x: number; }; }' is not assignable to type 'U$null'. Types of property 'kind' are incompatible. Type '"B"' is not assignable to type '"C" | "D"'.

I would expect the type of the argument to have been narrowed to `U$B` by checking the `kind` property first.  As it stands, the developer UX is not ideal especially when the shapes of the types involved get larger and it gets quite hard to figure out which property is wrong (in my real-world case, the union has 250+ members and the error message is incomprehensible).

**Notes:**
If I change `x` to `y` in `interface B`, the error message comes up OK.
Also, if I comment out the two references to `U$null`, I get a different (but also acceptable) error.

**Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior -->
https://www.typescriptlang.org/v2/en/play?ts=next#code/JYOwLgpgTgZghgYwgAgILIN7IB4C5kgCuAtgEbTIC+AUKJLIigEKY74DOYUoA5ldbXDR4SZAFUAJOiwBrUABN8AclRKANMlIB7eQE986GnWGNxElrIXKm6zTv3IWRoQ1GSiAGw+s5IRciUAYSVkAB8AgBFbbT0AfnxCPwgYUAh5fmowXQAHFDFkAF4zdHDJFlKJTw8AbgF5CAQPOCgUGESEMGAtEGQYLS0ACkJ8MQBKfAA3LWB5Wuo+wYxffxVo+3wsPGQAFgAmKkpR2oWBpasAmw0Yh038PYOj6iA

**Related Issues:** <!-- Did you find other bugs that looked similar? -->
None that (at an uneducated guess) seemed to be exactly this one.