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module-level const/let reads before the declaration don't throw (no TDZ at module level) #11826
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Correction to the diagnosis above, from a closer look on main
0a5bb47dc. The compound-assignment lowering is not at fault: it spills the base before the key. The real gap is that module-levelconst/letbindings have no TDZ at all. A read before the declaration yieldsundefinedinstead of throwing, so the compound form is only one symptom.At module level (node throws ReferenceError in every case):
const y = tdz;before the declaration: no throw.tdz.a,tdz["a"] += 1,tdz.a += 1,tdz[f()],tdz[f()] = 1,tdz[f()] += 1: TypeError, and the forms with a key call runf.() => tdzcalled before the declaration: returnsundefined.
Inside a function body every one of these forms already throws ReferenceError with zero key calls, so function-level TDZ works.
A fix needs TDZ for module-level lexical bindings. That means either a static rule (a read lexically before the declarator, at the same function level, throws) plus a check for closures that can run early, or a TDZ cell per module-level binding, which puts a cost on every global read. A prior attempt, #9762, was reverted in
1d63fa91f. Related: #6062, #6264, #6523, #11086. Retitling to match.- changed the title
[-]tdz[f()] += 1 on a const in its TDZ runs the key and throws TypeError instead of ReferenceError[/-][+]module-level const/let reads before the declaration don't throw (no TDZ at module level)[/+]on Oct 4, 2026 - added a commit that references this issue
on Oct 4, 2026 - added a commit that references this issue
on Oct 4, 2026
In a compound member assignment whose base is a
constbinding still in its temporal dead zone, perry evaluates the computed key first and then throws a TypeError. Per spec, the base is evaluated first, so reading the binding throws a ReferenceError before the key expression runs.Repro (main
34acf64e, v0.5.1656, Linux x86_64):ReferenceError f calls: 0ReferenceError f calls: 0TypeError f calls: 1Two differences: the key's side effect (
f()) runs when it shouldn't, and the error type is wrong. The TDZ check on the base is either skipped or ordered after the key spill in the compound-assignment lowering (hoist_compound_member_assign,crates/perry-hir/src/lower/expr_assign.rs).Found while building #11825, which doesn't change this behaviour: that PR keeps the base temp, ahead of the key, whenever the key is spilled.