[Sema] Implement type join for existential types.

Author: hborla

lib/AST/TypeJoinMeet.cpp

@@ -66,6 +66,7 @@ struct TypeJoin : CanTypeVisitor<TypeJoin, CanType> {
   CanType visitBoundGenericStructType(CanType second);
   CanType visitMetatypeType(CanType second);
   CanType visitExistentialMetatypeType(CanType second);
+  CanType visitExistentialType(CanType second);
   CanType visitModuleType(CanType second);
   CanType visitDynamicSelfType(CanType second);
   CanType visitArchetypeType(CanType second);
@@ -271,6 +272,29 @@ CanType TypeJoin::visitExistentialMetatypeType(CanType second) {
   return ExistentialMetatypeType::get(joinInstance)->getCanonicalType();
 }
 
+CanType TypeJoin::visitExistentialType(CanType second) {
+  assert(First != second);
+
+  if (First->getKind() != second->getKind())
+    return TheAnyType;
+
+  auto firstConstraint = First->castTo<ExistentialType>()
+      ->getConstraintType()->getCanonicalType();
+  auto secondConstraint = second->castTo<ExistentialType>()
+      ->getConstraintType()->getCanonicalType();
+
+  auto joinInstance = join(firstConstraint, secondConstraint);
+  if (!joinInstance)
+    return CanType();
+
+  if (joinInstance->is<ExistentialMetatypeType>() ||
+      joinInstance->isAny() ||
+      joinInstance->isAnyObject())
+    return joinInstance;
+
+  return ExistentialType::get(joinInstance)->getCanonicalType();
+}
+
 CanType TypeJoin::visitModuleType(CanType second) {
   assert(First != second);
 

test/Sema/type_join.swift

@@ -1,4 +1,5 @@
 // RUN: %target-typecheck-verify-swift -parse-stdlib
+// RUN: %target-typecheck-verify-swift -parse-stdlib -enable-explicit-existential-types
 
 import Swift