Sema: Try harder to avoid tautological inference by looking at reduced types

If a requirement `f(_: Self.Element)` has a default implementation
`f(_: Self.Iterator.Element)` where `Self: Sequence` for example,
we don't want to consider the witness as a source of inference,
because `Self.Element == Self.Iterator.Element` is always true.

Compute the reduced type of the witness, so that we can rule out
such cases using the existing logic.

Author: slavapestov

lib/Sema/TypeCheckProtocolInference.cpp

@@ -662,6 +662,18 @@ static Type getWitnessTypeForMatching(NormalProtocolConformance *conformance,
                              genericFn->getExtInfo());
   }
 
+  auto &ctx = conformance->getDeclContext()->getASTContext();
+
+  // Get the reduced type of the witness. This rules our certain tautological
+  // inferences below.
+  if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
+    if (auto genericSig = witness->getInnermostDeclContext()
+            ->getGenericSignatureOfContext()) {
+      type = genericSig.getReducedType(type);
+      type = genericSig->getSugaredType(type);
+    }
+  }
+
   // Remap associated types that reference other protocols into this
   // protocol.
   auto proto = conformance->getProtocol();