Fix potential soundness hole when adding references to a mapped capture set #18758
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odersky
added
the
cc-experiment
Fix soundness hole when adding references to a set that is the image of an
idempotent `tm` map `tm`. If the element `ref` did not come from the source
of the set, we still assumed that `tm(ref) = ref`, so that we simply added
the reference to the set and also back-propagated it to source. But that is not
necessarily the case (although it is the case in our complete test suite,
so I am not sure this case can actually arise in practice. Nevertheless,
it's better to not leave a potential soundness hole here.
In the new implementation, we test whether `tm(ref) = ref`, and only proceed
as before if that's the case. If not there are two sub-cases:
- `{ref} <:< tm(ref)` and the variance of the set is positive. In that case we
can soundly add `tm(ref)` to the set while back-propagating `ref` as before.
- Otherwise there's nothing obvious left to do except fail (which is always
sound.
odersky
changed the title
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Fix potential soundness hole when adding references to a set that is the image of an
idempotent
tmmaptm. If the elementrefdid not come from the sourceof the set, we still assumed that
tm(ref) = ref, so that we simply addedthe reference to the set and also back-propagated it to source. But that is not
necessarily the case. (Although it is the case in our complete test suite,
so I am not sure this case can actually arise in practice. Nevertheless,
it's better to not leave a potential soundness hole here.)
In the new implementation, we test whether
tm(ref) = ref, and only proceedas before if that's the case. If not there are two sub-cases:
{ref} <:< tm(ref)and the variance of the set is positive. In that case wecan soundly add
tm(ref)to the set while back-propagatingrefas before.sound).