Three more associated type inference fixes #71539
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…tological Fixes rdar://problem/122584912.
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A minimal Sequence conformance only needs to define an Iterator type, with the Element type witness inferred from the Element of the iterator. This trick didn't always work if the conforming type conformed to other protocols with declared same-type requirements involving Self.Element. Refine the heuristic introduced in 23599b6 to prefer abstract type witnesses in the current protocol, even if there is a shorter one in another protocol. Fixes rdar://problem/122574126, rdar://problem/122588328.
This is part of the -disable-experimental-associated-type-inference code path for abstract type witnesses, but I want to use it in another place. - Look in local conformances only, instead of all protocols. Philosophically, we want associated type inference to reason about each nominal type declaration or extension as a single unit. - Look for an abstract witness in any protocol that declares an associated type of the same name, not just a protocol that inherits from the associated type's protocol.
If a protocol fixes an associated type of another protocol via a concrete same-type requirement, we should make use of this fact immediately when building the associated type inference constraint system. Previously we only introduced fixed type bindings at the end, and only in the 'abstract type witness' inference path, which we reach if a type witness cannot be resolved by looking at value witnesses alone. This meant that associated type inference would find valid or ambiguous solutions which would then be contradicted by the ensureRequirementsAreSatisfied() check. Longer term, what I want to do is actually build the type witness system upfront, and also teach the constraint solver here about merging equivalence classes, so that we can split them again when backtracking. This will combine the best of both approaches. Fixes rdar://problem/122586992.
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Fixes rdar://problem/122584912, rdar://problem/122574126, rdar://problem/122586992, rdar://problem/122588328 (I count four fixes, not three, but whatever).