Malte UblandClaude Opus 5.5 1ef31a1b74 Continue a failed checked cast with its value in a dynamic tail ...

`JSON.parse(text) as T`, `const x: T = anyValue` and `await res.json()
as T` threw `TypeError: expected X at $.path, got Y` when the value did
not fit `T`. Node never checks an assertion and continues with the value
it has, so the build-container listener rejected an order Node accepts
and a JWT with a string `exp` became a 500.

A failed cast now keeps its value when it initializes a `const`/`let`
binding declared in a function or block and only the code after the cast
reads the binding. That code (the statements after the cast in each
enclosing statement list, up to the declaration) lowers twice: the
static copy, unchanged, runs while the value fits; a dynamic copy, in
which the binding and everything derived from it is typed `any`, runs
after a failure, so missing members read undefined, wrong-typed members
keep their values, extra members stay visible in source order, and
objects keep their identity. The cast sets the flag that selects the
copy: the fused JSON.parse helper's slow path and the plain check call
scr_dyn_check_recover, which takes back the TypeError and parks the
value for checkedCast.fallbackTaken/fallbackTake.

A failed value that flows into a typed slot outside the region is
checked there, as any `any` value is: it continues when it fits and
throws at that use when it does not. A statement of the dynamic copy
with no `any` lowering becomes a runtime fence that throws only when a
failed value reaches it. Module-level bindings, regions with hoisted
declarations or super() calls, union targets and targets holding class instances, and
--tier=static-only (and library mode) keep the throwing cast.

`a?.b.c` on an `any` value now short-circuits only on `a`, so `.c`
throws when `a.b` is undefined, as in Node.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2026-10-20 15:25:33 -07:00

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Continue a failed checked cast with its value in a dynamic tail

2026-10-20 15:25:33 -07:00

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Continue a failed checked cast with its value in a dynamic tail

2026-10-20 15:25:33 -07:00