Buffer: a dynamic (any-typed) string key silently writes a byte instead of an own property

[proggeramlug]

Storing to a Buffer/Uint8Array through a computed key that is a string at runtime but any at compile time silently writes a byte instead of an own property, and the value reads back undefined.

Repro

const buf = Buffer.alloc(4);

const keys: any[] = ["dyn"];
const k: any = keys[0];        // a string at runtime, `any` statically
(buf as any)[k] = "D";
console.log((buf as any)[k]);  // node: "D"   perry: undefined

Static string keys are fine (buf["lit"], or a local declared : string) — #6406 fixed those. This is the dynamic half, and it's the common shape: a key off an untyped array, out of Object.keys(), or returned from a call.

Root cause

The byte-path guard (expr_assign.rs, and mirrored in expr_member/member_tail.rs) folds to Uint8ArraySet / Uint8ArrayGet whenever the key is not provably a string:

let key_is_string = matches!(index.as_ref(), Expr::String(_))
    || matches!(index.as_ref(), Expr::LocalGet(kid) if /* declared Type::String */);
if !key_is_string && matches!(ty, Type::Named(n) if n == "Uint8Array" || n == "Buffer") { /* fold to bytes */ }

An any-typed key is not provably a string, so it folds — and the byte path coerces it to an index, dropping the property store.

Why the obvious fix doesn't work

Inverting the guard to require a provably numeric key is unsound-free but destroys the buffer fast path, because a for loop counter is not typed Number:

Let { id: 2, name: "i", ty: Any, mutable: true, init: Some(Integer(0)) }

A loop counter and a string-holding any key are the same type to this guard. Requiring Type::Number therefore un-folds every buf[i] in a loop. Measured on a plain for (let i…) buf[i] = i & 0xff kernel:

guard	Uint8Array folds
!key_is_string (today)	1
key_is_numeric	0

The native-region-proof compiler-output gate catches this immediately (its fixtures — h1_buffer_alias_negative, image_convolution — are exactly this shape). I tried it on #6406 and reverted.

Two real fixes

1. Infer Number for numeric induction variables. for (let i = 0; …; i++) with no non-numeric assignment in the body is a number. Then a sound key_is_numeric guard keeps the fast path and fixes this bug. Worth doing carefully — widening type visibility can wake latent fast paths that were only "correct" because unreachable (that's how perf(codegen): keep the numeric-array specialization when the array is captured (#6369) #6377 revived runtime: await Promise.all([...]) never resumes when inputs settle via executor-captured resolvers — program silently exits 0 #6328).
2. Discriminate at runtime. Always fold, and have the byte-path helper route a non-index key to the property path. Costs a check on the hot path but is robust to any key shape.

(1) is the cleaner win if the induction-variable analysis is cheap, since it also improves typing generally.