[4.1][CSBindings] Form bindings correctly when they come from 'OptionalObject' constraint

lib/Sema/CSBindings.cpp

@@ -402,6 +402,15 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
     if (type->hasError())
       continue;
 
+    // If the source of the binding is 'OptionalObject' constraint
+    // and type variable is on the left-hand side, that means
+    // that it _has_ to be of optional type, since the right-hand
+    // side of the constraint is object type of the optional.
+    if (constraint->getKind() == ConstraintKind::OptionalObject &&
+        kind == AllowedBindingKind::Subtypes) {
+      type = OptionalType::get(type);
+    }
+
     // If the type we'd be binding to is a dependent member, don't try to
     // resolve this type variable yet.
     if (type->is<DependentMemberType>()) {

test/Constraints/diagnostics.swift

@@ -1097,7 +1097,7 @@ func rdar17170728() {
 
   let _ = [i, j, k].reduce(0 as Int?) {
     $0 && $1 ? $0! + $1! : ($0 ? $0! : ($1 ? $1! : nil))
-    // expected-error@-1 {{type of expression is ambiguous without more context}}
+    // expected-error@-1 {{ambiguous use of operator '+'}}
   }
 }
 

test/Constraints/optional.swift

@@ -283,3 +283,8 @@ func testInOutOptionality() {
 
   overloadedByOptionality(&o)
 }
+
+// rdar://problem/37508855
+func rdar37508855(_ e1: X?, _ e2: X?) -> [X] {
+  return [e1, e2].filter { $0 == nil }.map { $0! }
+}