Sema: Teach getReferencedAssociatedTypes() to look through typealiases

Now that NameAliasTypes desugar to interface types, it is possible
to have a protocol requirement type contain a NameAliasType which
contains an associated type:

protocol P {
  associatedtype Element
  typealias Elements = [Element]

  func process(elements: Elements)
}

In Swift 3, the typealias would be desugared at name lookup time
in this case, but this is no longer the case, as a result associated
type inference stopped working in this example.

Fixes <https://bugs.swift.org/browse/SR-3641>.

lib/Sema/TypeCheckProtocol.cpp

@@ -506,13 +506,10 @@ static SmallVector<TupleTypeElt, 4> decomposeIntoTupleElements(Type type) {
 static AssociatedTypeDecl *
 getReferencedAssocTypeOfProtocol(Type type, ProtocolDecl *proto) {
   if (auto dependentMember = type->getAs<DependentMemberType>()) {
-    if (auto genericParam 
-          = dependentMember->getBase()->getAs<GenericTypeParamType>()) {
-      if (genericParam->getDepth() == 0 && genericParam->getIndex() == 0) {
-        if (auto assocType = dependentMember->getAssocType()) {
-          if (assocType->getDeclContext() == proto)
-            return assocType;
-        }
+    if (auto assocType = dependentMember->getAssocType()) {
+      if (dependentMember->getBase()->isEqual(proto->getSelfInterfaceType()) &&
+          assocType->getProtocol() == proto) {
+        return assocType;
       }
     }
   }
@@ -2158,7 +2155,7 @@ ConformanceChecker::getReferencedAssociatedTypes(ValueDecl *req) {
   // signature.
   auto &assocTypes = ReferencedAssociatedTypes[req];
   llvm::SmallPtrSet<AssociatedTypeDecl *, 4> knownAssocTypes;
-  req->getInterfaceType().visit([&](Type type) {
+  req->getInterfaceType()->getCanonicalType().visit([&](Type type) {
       if (auto assocType = getReferencedAssocTypeOfProtocol(type, Proto)) {
         if (knownAssocTypes.insert(assocType).second) {
           assocTypes.push_back(assocType);

test/decl/protocol/req/associated_type_inference.swift

@@ -343,3 +343,15 @@ extension Cookie {
 
 struct Thumbprint : Cookie {}
 // expected-error@-1 {{type 'Thumbprint' does not conform to protocol 'Cookie'}}
+
+// Looking through typealiases
+protocol Vector {
+  associatedtype Element
+  typealias Elements = [Element]
+
+  func process(elements: Elements)
+}
+
+struct Int8Vector : Vector {
+  func process(elements: [Int8]) { }
+}