Merge pull request #28626 from xedin/rdar-57668763

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

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -2660,6 +2660,17 @@ 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
+  }
+}