[ConstraintSystem] Fix a case of too eager force optional unwrap with optional contextual type

If return of optional chaining is assigned to an optional type
or discarded there should be no force optional unwrap attempt
on it because type variable associated with context can be
bound to optional of any depth.

Resolves: rdar://problem/57668873

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -2662,6 +2662,17 @@ repairViaOptionalUnwrap(ConstraintSystem &cs, Type fromType, Type toType,
   if (fromUnwraps <= toUnwraps && !fromObjectType->is<TypeVariableType>())
     return false;
 
+  // If the result of optional chaining is converted to
+  // an optional contextual type represented by a type
+  // variable e.g. `T?`, there can be no optional mismatch
+  // because `T` could be bound to an optional of any depth.
+  if (isa<OptionalEvaluationExpr>(anchor) && toUnwraps > 0) {
+    auto last = locator.last();
+    if (last && last->is<LocatorPathElt::ContextualType>() &&
+        toObjectType->is<TypeVariableType>())
+      return false;
+  }
+
   auto result =
       cs.matchTypes(fromObjectType, toObjectType, matchKind,
                     ConstraintSystem::TypeMatchFlags::TMF_ApplyingFix, locator);

test/Constraints/optional.swift

@@ -402,3 +402,21 @@ func sr_11104() {
   bar(["hello"].first)
   // expected-error@-1 {{cannot convert value of type 'String?' to expected argument type 'Int'}}
 }
+
+// rdar://problem/57668873 - Too eager force optional unwrap fix
+
+@objc class Window {}
+
+@objc protocol WindowDelegate {
+  @objc optional var window: Window? { get set }
+}
+
+func test_force_unwrap_not_being_too_eager() {
+  struct WindowContainer {
+    unowned(unsafe) var delegate: WindowDelegate? = nil
+  }
+
+  let obj: WindowContainer = WindowContainer()
+  if let _ = obj.delegate?.window { // Ok
+  }
+}