Applying a substitution list to a Substitution is done in two
steps:
1) First, apply the substitution list to the original replacement
type R to get the substituted replacement type R'.
2) Second, for each abstract conformance of R from the original
substitution, look up a concrete conformance of R' from the
substitution list.
With minimized generic signatures, we would omit conformances of
a nested type T.A if they could be derived from some other
conformance already in the substitution list.
However, the derivation process was incorrect, because it would
only look at the canonical parent type T, and not any other parent
type that had a child A which was same-type equivalent to T.A.
For example, consider the following code:
protocol P1 { associatedtype A : P3 }
protocol P2 { associatedtype A : P4 }
protocol P3 {}
protocol P4 {}
func doSomething<T : P4>(...) {}
func doSomethingElse<T1 : P1, T2 : P2>(...) where T1.A == T2.A {
doSomething(...)
}
If we specialize doSomethingElse() with a pair of concrete types
to replace T1 and T2, we would need to find a concrete conformance
to replace the abstract conformance T2.A : P4 in the call to
doSomething().
Since the conformance of T2.A : P4 is a redundant requirement,
it does not appear in the conformance map; furthermore, T1.A and
T2.A are same-type equivalent, so they map to the same archetype.
We would therefore look at the canonical parent of T2.A, which is
T1, and look up the conformance of T1 : P1 in the substitution list.
However, the only requirement P1 imposes on A is the conformance
A : P3. There's no conformance A : P4 inside T1 : P1, so we would
hit an assertion.
Indeed, the conformance T1.A : P4 must be recovered from the
conformance of T2 : P2, because P2 requires that A : P4.
This patch ensures that the conformance can be found by changing
the ArchetypeConformanceMap from a simple mapping of archetypes
to conformances into a structure that records same-type constraints
as well.
So instead of just looking at the canonical parent of the archetype
T1.A, we consult the structure to check all archetypes that have
T1.A as a child, in this case both T1 and T2.
T2 : P2 contains the conformance we need, allowing the above code
to be specialized correctly.
These will be more useful once substitutions in protocol conformances
are moved to use interface types.
At first, these are only going to be used by the SIL optimizer.
This patch is rather large, since it was hard to make this change
incrementally, but most of the changes are mechanical.
Now that we have a lighter-weight data structure in the AST for mapping
interface types to archetypes and vice versa, use that in SIL instead of
a GenericParamList.
This means that when serializing a SILFunction body, we no longer need to
serialize references to archetypes from other modules.
Several methods used for forming substitutions can now be moved from
GenericParamList to GenericEnvironment.
Also, GenericParamList::cloneWithOuterParameters() and
GenericParamList::getEmpty() can now go away, since they were only used
when SILGen-ing witness thunks.
Finally, when printing generic parameters with identical names, the
SIL printer used to number them from highest depth to lowest, by
walking generic parameter lists starting with the innermost one.
Now, ambiguous generic parameters are numbered from lowest depth
to highest, by walking the generic signature, which means test
output in one of the SILGen tests has changed.
A GenericEnvironment stores the mapping between GenericTypeParamTypes
and context archetypes (or eventually, concrete types, once we allow
extensions to constrain a generic parameter to a concrete type).
The goals here are two-fold:
- Eliminate the GenericTypeParamDecl::getArchetype() method, and
always use mapTypeIntoContext() instead
- Replace SILFunction::ContextGenericParams with a GenericEnvironment
This patch adds the new data type as well as serializer and AST
verifier support. but nothing else uses it yet.
Note that GenericSignature::get() now asserts if there are no
generic parameters, instead of returning null. This requires a
few tweaks here and there.
