There was a bunch of logic to lazily populate replacement types
corresponding to reducible generic parameters. This didn't seem
to have a clear purpose so let's remove it.
Start treating the null {Can}GenericSignature as a regular signature
with no requirements and no parameters. This not only makes for a much
safer abstraction, but allows us to simplify a lot of the clients of
GenericSignature that would previously have to check for null before
using the abstraction.
Structurally prevent a number of common anti-patterns involving generic
signatures by separating the interface into GenericSignature and the
implementation into GenericSignatureBase. In particular, this allows
the comparison operators to be deleted which forces callers to
canonicalize the signature or ask to compare pointers explicitly.
When forming a substitution map from a type substitution function and
conformance lookup function, we were calling the type substitution function
for generic parameters that aren't canonical (e.g., because they are
same-typed to something else). The type substitution functions wouldn't
necessarily take the new generic signature into account, so we would get
incorrect replacement types.
Detect non-canonical generic parameters when forming substitution maps,
and leave those entries in the "replacements" array blank; we'll fill them
in later, properly, when needed.
Introduced during the bring-up of the generics system in July, 2012,
Substitution (and SubstitutionList) has been completely superseded by
SubstitutionMap. R.I.P.
Every SubstitutionMap::toList() invocation would ASTContext-allocate
arrays for all of the conformances, then return a SmallVector of the
underlying substitutions, which both wastes memory *and* puts the onus
on the caller to allocate a copy of that outer SmallVector into the
ASTContext if it's going to be stored anywhere.
Stop the madness by caching an ASTContext-allocated SubstitutionList
within the storage for the SubstitutionMap. This both reduces repeated
allocations and eliminates the potential for errors.
Now that SubstitutionMap is used in so many places, reduce it's header
dependencies by moving SubstitutionMap::Storage into its own separate
implementation header. Use forward declarations of other entities
(GenericSignature, Substitution) instead.
Good for build times and general sanity.
