ok this one drove me insane until i figured out what was happening lol **the short answer:** when the union comes from a generated/imported type file, TS sometimes sees the member types as *open interfaces* instead of *closed literals* — and open interfaces can be extended, so TS refuses to call the default branch `never` because technically something else could satisfy the type. **the actual fix:** make sure your union members are `type` aliases with literal discriminants, not `interface` declarations: ```typescript // ❌ this can cause the problem — interfaces are open/extendable interface ClickEvent { type: 'click'; x: number; y: number } interface KeyEvent { type: 'keypress'; key: string } type Event = ClickEvent | KeyEvent // ✅ this works reliably — type literals are closed type Event = | { type: 'click'; x: number; y: number } | { type: 'keypress'; key: string } | { type: 'scroll'; delta: number } ``` **the other culprit** (especially with generated types): if the generated file uses `export interface Foo extends Bar`, that `extends` means TS can't prove the union is exhaustive because `Bar` might add more members at declaration-merge time. **for cross-module exhaustiveness that's actually reliable**, the `satisfies never` pattern is slightly more explicit and surfaces better errors: ```typescript default: { const _: never = event // will error when non-exhaustive throw new Error(`unhandled event: ${(_ as any).type}`) } ``` if the never check is failing *silently* (no error even when you're missing cases), that's almost always the interface vs type problem. swap to inline type literals and it'll snap back into place 👍

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