Discriminant-specific error message not shown when one constituent of a union has a union-typed discriminant

TypeScript Version: 3.9.0-dev.20200319

Search Terms: discrimant tagged union assignable

Code

A simplified repro, based on real-world code:

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:
https://www.typescriptlang.org/v2/en/play?ts=next#code/JYOwLgpgTgZghgYwgAgILIN7IB4C5kgCuAtgEbTIC+AUKJLIigEKY74DOYUoA5ldbXDR4SZAFUAJOiwBrUABN8AclRKANMlIB7eQE986GnWGNxElrIXKm6zTv3IWRoQ1GSiAGw+s5IRciUAYSVkAB8AgBFbbT0AfnxCPwgYUAh5fmowXQAHFDFkAF4zdHDJFlKJTw8AbgF5CAQPOCgUGESEMGAtEGQYLS0ACkJ8MQBKfAA3LWB5Wuo+wYxffxVo+3wsPGQAFgAmKkpR2oWBpasAmw0Yh038PYOj6iA

Related Issues:
None that (at an uneducated guess) seemed to be exactly this one.

Beginne mit der bereitgestellten TypeScript Playground-Reproduktion und vergleiche die für den zweiten foo-Aufruf erzeugte Diagnose mit dem erwarteten diskriminantenspezifischen Fehler. Verfolge den Pfad im Type-Checker, der Unions mit union-typisierten Diskriminanten behandelt; abgeschlossen ist die Arbeit, wenn der Fehler auf x: 42 verweist und meldet, dass number nicht string zugewiesen werden kann, ohne die genannten Varianten zu beeinträchtigen.