Dynamic (n as any).toString() ignores the exponential threshold: 2.2e-308 prints ~308 decimal digits (static lowering is correct) #9713

Found while validating #9698. Not caused by it — see "Why this is not #9698" below.

Repro

const a: number = 2.2e-308;

// Static lowering — all correct.
console.log(a.toString());   // 2.2e-308
console.log(String(a));      // 2.2e-308
console.log(`${a}`);         // 2.2e-308
console.log(a);              // 2.2e-308

// Dynamic dispatch — wrong.
function dyn(x: any, m: string): any { return x[m](); }
console.log(dyn(a, "toString"));

The four static forms and the dynamic one disagree in the same program, so
this is a dispatch-path divergence, not a formatter that is simply wrong.

Where it is not

string::format::js_format_f64 implements the ECMA-262 NumberToString
threshold correctly — scientific notation when |n| >= 1e21 or |n| < 1e-6,
with a comment explaining exactly why (Number.EPSILON otherwise prints a
16-digit 0.000…0002…). js_number_to_string calls it, and the radix-10 arm
of value::to_string::number_to_radix_string returns js_number_to_string(n)
directly. So whatever the dynamic tower does for this receiver, it is not
reaching that formatter — the divergence is upstream of it.

I did not localise it further, and I am deliberately not guessing: the
primitive_methods toString arms I read are all jsval.is_pointer()-gated,
so a plain-number receiver does not take them, and I ran out of verified ground
before finding the arm that answers.

A measured localisation clue

Two subnormals, same zero-argument dynamic .toString(), different results —
and they reach Number.prototype by different routes:

1e-310 is bare-shaped (bits >> 48 == 0), 2.2e-308 is not
(bits >> 48 == 15), which is the only reason they diverge in routing. So the
route that goes straight to call_primitive_builtin_prototype_method formats
correctly, and the ordinary tower's route does not. That should narrow the
search considerably.

Scope — not determined

I verified 2.2e-308 only. Untested, and worth checking as part of a fix,
is whether the same dynamic path also mishandles the other side of the
threshold and the ordinary-magnitude exponential cases:

• 1e21, 1e-7, Number.MAX_VALUE, Number.MIN_VALUE, Number.EPSILON
• the same values via String(x) / `${x}` / + "" reached dynamically
• non-10 radices (dyn(x, "toString", 16) on the same values)

dyn(255, "toString", 16) → "ff" and dyn(3.14159, "toFixed", 2) → "3.14"
are correct, so the argument-carrying dynamic forms are at least not
uniformly broken.

Reproducing

The environment I measured in has been torn down. To rebuild:

cargo build --release -p perry
cargo build --release -p perry-runtime-static -p perry-stdlib-static
./target/release/perry compile repro.ts -o repro && ./repro
node --experimental-strip-types repro.ts

Severity: low on its own — a number stringifies to a mathematically correct but
non-conforming form. It matters where the text is compared or parsed (JSON-ish
output, cache keys, snapshot tests, log diffing), and it is a parity divergence
that only appears under type erasure, which is exactly the shape minified
bundles produce.