[Associated type inference] Don’t get blocked by typealiases in protocol extensions.

Typealiases in protocol extensions can be used to satisfy 
associated type requirements. However, when they don’t meet all
of the requirements placed on the associated type, fall back to
the normal inference path rather than failing outright.

Fixes SR-6609 / rdar://problem/36038033.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -1795,8 +1795,8 @@ void ConformanceChecker::recordTypeWitness(AssociatedTypeDecl *assocType,
 
   // If we already recoded this type witness, there's nothing to do.
   if (Conformance->hasTypeWitness(assocType)) {
-    assert(Conformance->getTypeWitness(assocType, nullptr)
-             ->isEqual(type) && "Conflicting type witness deductions");
+    assert(Conformance->getTypeWitness(assocType, nullptr)->isEqual(type) &&
+           "Conflicting type witness deductions");
     return;
   }
 
@@ -2750,6 +2750,16 @@ ResolveWitnessResult ConformanceChecker::resolveTypeWitnessViaLookup(
     }
   }
 
+  // If there are no viable witnesses, and all nonviable candidates came from
+  // protocol extensions, treat this as "missing".
+  if (viable.empty() &&
+      std::find_if(nonViable.begin(), nonViable.end(),
+                   [](const std::pair<TypeDecl *, CheckTypeWitnessResult> &x) {
+                     return x.first->getDeclContext()
+                        ->getAsProtocolOrProtocolExtensionContext() == nullptr;
+                   }) == nonViable.end())
+    return ResolveWitnessResult::Missing;
+
   // If there is a single viable candidate, form a substitution for it.
   if (viable.size() == 1) {
     auto interfaceType = viable.front().second;

test/decl/protocol/req/associated_type_inference.swift

@@ -469,3 +469,19 @@ protocol P17 {
 protocol Q17 : P17 where T == Int { }
 
 struct S17 : Q17 { }
+
+// Typealiases from protocol extensions should not inhibit associated type
+// inference.
+protocol P18 {
+  associatedtype A
+}
+
+protocol P19 : P18 {
+  associatedtype B
+}
+
+extension P18 where Self: P19 {
+  typealias A = B
+}
+
+struct X18<A> : P18 { }