typed throwsやるなら、同時にuntagged union ( T|U ) の整備も必要だと思うのだけど、そういう指摘はあるかしら

typed throwsと別のスレッドができましたね。(Coreチーム発ではないですが) https://forums.swift.org/t/structural-sum-types-used-to-be-anonymous-union-types/67432 (edited)

Structural Sum Types (used to be Anonymous Union Types)

Spinning off a new discussion thread on the topic of anonymous union types from the Status check: Typed throws thread. The spin-off discussion on this topic began somewhere around this comment.