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CallTypedArrayMethodIfWrapped (and the CallNonGeneric machinery throughout the
engine) unwraps the `this` argument, but the other arguments are only rewrapped
for the target compartment.
The pattern being used before this patch to get the length of a TypedArray or
possible TypedArray wrapper is:
`callFunction(CallTypedArrayMethodIfWrapped, O, O, "TypedArrayLength")`
The first `O` is the `this` value and the second is an argument. If `O` is a
cross-compartment wrapper, this works fine. The first `O` is unwrapped, revealing
the actual TypedArray object; the second `O` is rewrapped for that TypedArray's
compartment, producing the same TypedArray.
However, if `O` is a same-compartment wrapper, this doesn't work. The first `O` is
unwrapped, revealing the actual TypedArray object in the same compartment;
rewrapping the other `O` does nothing to it, since it is already an object in the
target compartment. Thus TypedArrayLength receives a `this` value that's an
unwrapped TypedArray, but an argument that is still a wrapper.
The fix is to have CallTypedArrayMethodIfWrapped targets only expect `this`
to be an unwrapped TypedArray.
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