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Malte Ubl
1c974f015a
A typed-array read that may name a missing element lowers to a boxed
`number | undefined`, and the optional-value analysis widens every
parameter it reaches for all callers. Two patterns in tsc-ts's relater
boxed its hottest relation methods this way, failing the build's scalar
ABI check ("Boxed numeric arguments in Checker.isTypeAssignableTo"):
- `const regular = this.regularTypes[id]; const sourceId = regular >= 0
? regular : id;` The read is NaN-coded, but its occurrence in the true
arm was rebuilt as `number | undefined`, so `sourceId` and the
parameters of isSimpleTypeRelatedTo and structuredTypeRelatedTo were
boxed. A comparison with a number that is never NaN is false for
undefined and NaN alike, so an occurrence of a never-reassigned binding
under such a guard is a plain, non-NaN number. The analysis now treats
it as one wherever it flows.
- `for (let i = 0; i < sources.length; i++) isTypeAssignableTo(sources[i],
targets[i])` over two Uint32Arrays allocated with the same length. The
parameters also receive a JSON field that may be NaN, so NaN-coding
cannot carry them (`source === target` must tell NaN from undefined).
The reads are now proven present instead: the index is a non-negative
integer counter that only its own function writes, a dominating guard
bounds it by the array's length or by the length of a parallel typed
array allocated with the same length, and each array is a `const`
fixed-length typed array referenced only through element accesses and
`.length`, so no code can detach its buffer. A proven-present read
lowers to the plain element read and is never optional.
indexReadInBounds also accepts a read inside an `if` condition and takes
an optional same-length fact. Reads the proofs do not cover keep the
established lowering: an inclusive bound, a shorter parallel array, a
decreasing or fractional counter, a counter a closure writes, and an
array that escapes all still read undefined, as the new corpus program
typed-array-present-reads checks against Node.