Function-expression implicit return leaves numeric sentinel in return slot instead of undefined

[proggeramlug]

Background

While addressing PR #754 review item 2b (throw on class decorator that returns a replacement class), the natural check would have been:

if ret !== undefined { throw TypeError(...) }

The PR author had to fall back to:

if typeof ret === "function" { throw TypeError(...) }

…with this inline comment in crates/perry-hir/src/lower.rs:

Check typeof ret === "function" rather than ret !== undefined: Perry's lowering for a function expression with no explicit return currently leaves a numeric sentinel in the return slot rather than the NaN-boxed undefined value, so !== undefined would false-positive on side-effect-only decorators.

So bare @Injectable (function expression with no return) returns some non-undefined value — a numeric sentinel — instead of the NaN-boxed undefined that strict JS semantics require.

Why this matters

1. It's masking a wider correctness bug. The decorator workaround only catches the typeof === "function" case. A decorator that returns a primitive or plain object would currently be silently discarded; once this is fixed, the decorator check can be tightened to the cleaner !== undefined shape and catch all replacement-attempt patterns.
2. Any user code that does if (someFn() !== undefined) against a function with no explicit return is silently wrong. That's a fundamental JS semantic and the workaround comment suggests Perry currently violates it for a specific lowering path.

Repro shape (suggested — needs verification)

function bare() { /* no return */ }
const r = bare();
console.log(r === undefined);          // node: true
console.log(typeof r);                  // node: "undefined"
console.log(r);                         // node: undefined

Worth confirming whether this reproduces standalone or only on the specific decorator-invocation lowering path that the PR #754 author hit.

Suggested fix

Audit Perry's function-expression epilogue lowering in crates/perry-codegen/ (probably the Stmt::Return insertion when the function body falls off the end without an explicit return) and ensure the implicit return slot is the NaN-boxed TAG_UNDEFINED value (0x7FFC_0000_0000_0001) rather than whatever numeric sentinel is currently leaking through.

Once fixed:

• Tighten the append_decorator_invocations_inner check from typeof ret === "function" to ret !== undefined so non-function class-decorator returns also throw.
• Remove the workaround comment.

Related

• PR Add legacy TypeScript decorator metadata support #754 — the workaround that surfaced this
• crates/perry-hir/src/lower.rs — append_decorator_invocations_inner, with the explanatory comment