symtable_cache_ptr now points to the first unused symtable_cache
entry, rahter than the last used one. This avoids taking a pointer
to the minus first element of the array, which is UB. Instead we
take a pointer to the end plus one, which is not UB.
Keep track of delayed variance obligations and check them after
linking a class is otherwise finished. Obligations may either be
unresolved method compatibility (because the necessecary classes
aren't available yet) or open parent/interface dependencies. The
latter occur because we allow the use of not fully linked classes
as parents/interfaces now.
An important aspect of the implementation is we do not require
classes involved in variance checks to be fully linked in order for
the class to be fully linked. Because the involved types do have to
exist in the class table (as partially linked classes) and we do
check these for correct variance, we have the guarantee that either
those classes will successfully link lateron or generate an error,
but there is no way to actually use them until that point and as
such no possibility of violating the variance contract. This is
important because it ensures that a class declaration always either
errors or will produce an immediately usable class afterwards --
there are no cases where the finalization of the class declaration
has to be delayed until a later time, as earlier variants of this
patch did.
Because variance checks deal with classes in various stages of
linking, we need to use a special instanceof implementation that
supports this, and also introduce finer-grained flags that tell us
which parts have been linked already and which haven't.
Class autoloading for variance checks is delayed into a separate
stage after the class is otherwise linked and before delayed
variance obligations are processed. This separation is needed to
handle cases like A extends B extends C, where B is the autoload
root, but C is required to check variance. This could end up
loading C while the class structure of B is in an inconsistent
state.
We weren't able to do this in 7.1 because the deprecation notice
may be converted to an exception and __toString() can't throw,
which means that it ultimately become a fatal error. This issue
is resolved now, so we can mark the method as deprecated.
