Design Meeting Notes, 2026-06-23

# `isolatedDeclarations` and Control Flow Analysis

https://github.com/microsoft/typescript-go/pull/4367

* Want to be able to infer types for trivial expressions in `isolatedDeclarations` in a function block.
* Specifically in discussion is if there are no return statements.
* But what happpens when you have a `never`-returning call? That can cause a `never`.
* Feels like anything with multiple statements in the body is questionable.
* We *could* make a special-case for `() => {}`.
* Probably enough to just say "just write `void`".
* Feels bad to say we can't come up with a type for `() => {}`
* Why? Feels like it's not a big deal?
* Well it's even more trivial than a single expression being returned.
* Sure, but the reason we added trivial expression inference was because people often have bigger structures of objects that are nested. So the triviality was not really about writing small expressions.
* One reason why we didn't do it (we think) is that we don't want to close the door to ourselves on how `void` vs. `undefined` is inferred for no-return-expression functions.

Start by reading the issue discussion and the linked pull request, microsoft/typescript-go#4367. Clarify the intended isolatedDeclarations behavior for function bodies with no return statements, including never-returning calls and the void-versus-undefined distinction. Done means the project has an agreed behavior and corresponding implementation scope.

Written by the indexing model from the issue text.