A protocol conformance needs to know what declarations satisfy requirements;
these are called "witnesses". For a value (non-type) witness, this takes the
form of a ConcreteDeclRef, i.e. a ValueDecl plus any generic specialization.
(Think Array<Int> rather than Array<T>, but for a function.)
This information is necessary to compile the conformance, but it causes
problems when the conformance is used from other modules. In particular,
the type used in a specialization might itself be a generic type in the
form of an ArchetypeType. ArchetypeTypes can't be meaningfully used
outside their original context, however, so this is a weird thing to
have to deal with. (I'm not going to go into when a generic parameter is
represented by an ArchetypeType vs. a GenericTypeParamType, partially
because I don't think I can explain it well myself.)
The above issue becomes a problem when we go to use the conformance from
another module. If module C uses a conformance from module B that has a
generic witness from module A, it'll think that the archetypes in the
specialization for the witness belong in module B. Which is just wrong.
It turns out, however, that no code is using the full specializations for
witnesses except for when the conformance is being compiled and emitted.
So this commit sidesteps the problem by just not serializing the
specializations that go with the ConcreteDeclRef for a value witness.
This doesn't fix the underlying issue, so we should probably still see
if we can either get archetypes from other contexts out of value witness
ConcreteDeclRefs, or come up with reasonable rules for when they're okay
to use.
rdar://problem/23892955
