[CSBindings] Delay binding chain result if type is an optional of a…

lib/Sema/CSBindings.cpp

@@ -743,6 +743,24 @@ ConstraintSystem::getPotentialBindingForRelationalConstraint(
       return None;
   }
 
+  // If subtyping is allowed and this is a result of an implicit member chain,
+  // let's delay binding it to an optional until its object type resolved too or
+  // it has been determined that there is no possibility to resolve it. Otherwise
+  // we might end up missing solutions since it's allowed to implicitly unwrap
+  // base type of the chain but it can't be done early - type variable
+  // representing chain's result type has a different l-valueness comparing
+  // to generic parameter of the optional.
+  if (kind == AllowedBindingKind::Subtypes) {
+    auto *locator = typeVar->getImpl().getLocator();
+    if (locator &&
+        locator->isLastElement<LocatorPathElt::UnresolvedMemberChainResult>()) {
+      auto objectType = type->getOptionalObjectType();
+      if (objectType && objectType->isTypeVariableOrMember()) {
+        result.PotentiallyIncomplete = true;
+      }
+    }
+  }
+
   if (type->is<InOutType>() && !typeVar->getImpl().canBindToInOut())
     type = LValueType::get(type->getInOutObjectType());
   if (type->is<LValueType>() && !typeVar->getImpl().canBindToLValue())

test/expr/delayed-ident/member_chains.swift

@@ -344,3 +344,30 @@ extension CurriedGeneric where T == Int {
 
 let _: CurriedGeneric = .createInt(CurriedGeneric())()
 let _: CurriedGeneric = .create(CurriedGeneric())(Int.self)
+
+// rdar://problem/68094328 - failed to compile unresolved member with implicit optional promotion
+func rdar68094328() {
+  struct S {
+    init(string: String) {}
+
+    var value: S {
+      get { S(string: "") }
+    }
+
+    func baz(str: String) -> S {
+      S(string: str)
+    }
+  }
+
+  class C {
+    func bar(_: S) {}
+  }
+
+  func foo<T>(_: (C) -> (T) -> Void, _: T?) {}
+
+  func test(str: String) {
+    foo(C.bar, .init(string: str)) // Ok
+    foo(C.bar, .init(string: str).value) // Ok
+    foo(C.bar, .init(string: str).baz(str: "")) // Ok
+  }
+}