ProposedNewObject intentionally replaces a null prior with an unknown
prior in order to easily fill in unknown values where they "show through"
under values not set explicitly in config, but it was failing to handle
that situation when dealing with nested blocks that are backed by sets.
We need to make the collection itself be a tuple or object rather than
list or map in this case, since otherwise all of the elements of the
collection are constrained to be of the same type and that isn't the
intent of a provider indicating that it accepts any type.
This produces a "proposed new state", which already has prior computed
values propagated into it (since that behavior is standard for all
resource types) but could be customized further by the provider to make
the "_planned_ new state".
In the process of implementing this it became clear that our configschema
DecoderSpec behavior is incorrect, since it's producing list values for
NestingList and map values for NestingMap. While that seems like it should
be right, we should actually be using tuple and object types respectively
to allow each block to have a different runtime type in situations where
an attribute is given the type cty.DynamicPseudoType. That's not fixed
here, and so without a further fix list and map blocks will panic here.
The DecoderSpec implementation will be fixed in a subsequent commit.
Martin Atkins