[ConstraintSystem] Return an existential type in getTypeOfMemberReference

when the member is a typealias to a protocol or composition.

Author: hborla

lib/Sema/ConstraintSystem.cpp

@@ -1736,6 +1736,13 @@ ConstraintSystem::getTypeOfMemberReference(
     auto memberTy = TypeChecker::substMemberTypeWithBase(DC->getParentModule(),
                                                          typeDecl, baseObjTy);
 
+    // If the member type is a constraint, e.g. because the
+    // reference is to a typealias with an underlying protocol
+    // or composition type, the member reference has existential
+    // type.
+    if (memberTy->isConstraintType())
+      memberTy = ExistentialType::get(memberTy);
+
     checkNestedTypeConstraints(*this, memberTy, locator);
 
     // Convert any placeholders and open any generics.

test/type/explicit_existential.swift

@@ -193,3 +193,12 @@ extension MyError {
     return true
   }
 }
+
+struct Wrapper {
+  typealias E = Error
+}
+
+func typealiasMemberReferences(metatype: Wrapper.Type) {
+  let _: Wrapper.E.Protocol = metatype.E.self
+  let _: (any Wrapper.E).Type = metatype.E.self
+}