Hopefully the codesize checks will smile upon me...

Yeah it's kind of... special... that ToWebAssemblyValue sends any length two or greater typed array to 0 but a length one typed array goes to the value inside.

Right, it calls ToInt32 which calls ToIntegerOrInfinity which ToNumber(ToPrimitive(x)). ToPrimitive(x) converts to a string and if the length is 1 ToNumber() converts it to the number inside and otherwise converts to NaN. Then ToIntegerOrInfinity converts the NaN to 0 and everything else passes through.

Right, it calls ToInt32 which calls ToIntegerOrInfinity which ToNumber(ToPrimitive(x)). ToPrimitive(x) converts to a string and if the length is 1 ToNumber() converts it to the number inside and otherwise converts to NaN. Then ToIntegerOrInfinity converts the NaN to 0 and everything else passes through.

And yet it won't coerse the number 0 to an i64 without bigint suffix on the 0n. Seems crazy.

Ah so wasmFunctionWhichExpectsBigInt(new Uint8Array(7)) sees a 7, but wasmFunctionWhichExpectsBigInt(7) is a type error. Right.

Ah so wasmFunctionWhichExpectsBigInt(new Uint8Array(7)) sees a 7, but wasmFunctionWhichExpectsBigInt(7) is a type error. Right.

Really, I would expect wasmFunctionWhichExpectsBigInt(new Uint8Array(7)) not to works too.. does it work?

It converts it to a string and ToBigInt happily converts a string to a bigint just not a number
https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tobigint

const bytes = new Uint8Array([
  0, 97, 115, 109, 1, 0, 0, 0,   // magic + version
  1, 6, 1, 96, 1, 126, 1, 127,   // type section: (i64) -> i32
  3, 2, 1, 0,                          // function section
  7, 8, 1, 4, 119, 114, 97, 112, 0, 0, // export "wrap"
  10, 7, 1, 5, 0, 32, 0, 167, 11,      // code: local.get 0; i32.wrap_i64; end
]);

const arr = new Uint8Array([200]);

WebAssembly.instantiate(bytes).then(({ instance }) => {
  const { wrap } = instance.exports;
  console.log("result:", wrap(arr)); // logs 200
});