[CSSimplify] Fix handling of holes by OptionalObject constraint

When "optional type" is a hole the simplification logic has to
simplify "object type" and mark all of its unresolved components
are potential holes, otherwise hole propagation won't work since
sometimes there is no other contextual information for such types
but "optional type".

Resolves: rdar://117871338

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -9184,8 +9184,8 @@ ConstraintSystem::simplifyOptionalObjectConstraint(
   }
 
   if (optTy->isPlaceholder()) {
-    if (auto *typeVar = second->getAs<TypeVariableType>())
-      recordPotentialHole(typeVar);
+    // object type should be simplified because it could be already bound.
+    recordAnyTypeVarAsPotentialHole(simplifyType(second));
     return SolutionKind::Solved;
   }
 

test/stmt/switch_stmt2.swift

@@ -150,3 +150,25 @@ func fallthrough_not_last(i: Int) {
     break
   }
 }
+
+// rdar://117871338 - incorrect diagnostic - type of expression is ambiguous when member is missing.
+func test_invalid_optional_chaining() {
+  func test(_: (E) -> Void) {
+  }
+
+  enum E {
+  case a
+  case b
+  }
+
+  struct S {
+    var prop: E
+  }
+
+  test {
+    switch $0.prop? { // expected-error {{value of type 'E' has no member 'prop'}}
+    case .a: break
+    case .b: break
+    }
+  }
+}