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.

Inizia con la riproduzione fornita in TypeScript Playground e confronta la diagnostica prodotta per la seconda chiamata a foo con l’errore atteso specifico del discriminante. Segui il percorso del type-checker che gestisce le unioni con discriminanti di tipo unione; il lavoro è completato quando l’errore punta a x: 42 e segnala che number non è assegnabile a string, senza causare regressioni nelle varianti indicate.