Moreover, the Completion record (either normal or abrupt) returned
from [[Call]] will hold an additional [[thisValue]] field,
which, unless otherwise specified:
* is set to the original `thisArgument` if it was not empty; or,
* is set to the value (at the time of completion) of the this-binding,
if it was initially uninitialised but has been initialised; or,
* is absent if the this-binding was left uninitialised.
Hmm, mixed feelings about extending the Completion record type to hold
another field. It could be problematic for implementors to store this
additional state efficiently. On second thought, it should be possible
to merge [[thisValue]] with the existing [[Value]] field, and
conditionally handle [[Value]] == empty in [[Construct]]? [1]
Moreover, the Completion record (either normal or abrupt) returned
from [[Call]] will hold an additional [[thisValue]] field,
which, unless otherwise specified:
* is set to the original `thisArgument` if it was not empty; or,
* is set to the value (at the time of completion) of the this-binding,
if it was initially uninitialised but has been initialised; or,
* is absent if the this-binding was left uninitialised.
Tail-call semantics may interfere with determining the value of
"this-binding at the time of completion", because the execution context
was already popped from the stack at that time. Probably solvable by
determining the this-binding before performing the tail-call, plus some
special casing when the tail-call expression is a super-call.
--------------------------------
When, say, `super.method(..args)` is called, the following steps are taken:
1. Let `F` be the method referenced by `super.method`.
This part needs to be redefined. `super.method` is an abbreviation for:
1. Let env be GetThisEnvironment().
2. Let baseValue be GetSuperBase(env).
3. Let actualThis be GetThisBinding().
4. Let result be baseValue.[[Get]]("method", actualThis).
But at `this`-binding initialisation time, `actualThis` is still the
empty placeholder object, so it cannot be used as the receiver argument
for the [[Get]] internal method call.
[1] https://gist.github.com/anba/e6b525c124d09dafaed6
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