A closure created above the `let`/`const` it captures holds the binding
in a TDZ box whose NULL payload slot marks "not yet initialized". A
union of one reference arm with `null` and/or `undefined` (`Set<T> |
null`, `string | undefined`, `C | null`) is represented as the bare arm
pointer with NULL standing for the unit arm, so storing that unit value
made the box indistinguishable from the temporal dead zone: every later
read or write threw "Cannot access 'x' before initialization" although
the declaration had run.
The self-hosted compiler trips on exactly this shape in
lowerer.ts (`let dynamicViews: Set<string> | null = null`, read by a
callback created earlier), which made scriptc 0.2.8 fail to build
tsc-ts's relater with SC3004. Node evaluates the source correctly; the
native code was wrong.
TDZ boxes of nullable-union bindings now start at the module's immortal
NULLABLE_ABSENT sentinel and the TDZ test compares against it. No value
of the binding can hold that sentinel, and every RC entry point and the
cycle collector already skip it, so box release, trace and the
initializing store need no other change.
Corpus: 4596-tdz-nullable-forward-capture.
Malte Ubl
5d0b38ef87