Strange type narrowing with Discriminated union types

[Lenne231]

TypeScript Version:

nightly (1.9.0-dev.20160619-1.0)

Expected behavior:

Type of shape should be never or Shape so that we can still access type to throw an Error.

  throw new Error('Unknown type. Type: ' + shape.type);

Actual behavior:

Type of shape is { type: 'rectangle'; width: number; height: number }.

[ahejlsberg]

Duplicate of #9246. This is working as intended because of #8548, but it is obviously causing some confusion.

[yortus]

@ahejlsberg how do the conditions of #8548 come up with this example? The control flow type is (or should be?) narrowed to never after the two type guards in the if/else if block. But then there is no third type guard that conflicts with that narrowed type, so why should #8548 apply?

[ahejlsberg]

Fixed in #10118.