diff --git a/lib/Sema/ConstraintSystem.cpp b/lib/Sema/ConstraintSystem.cpp
index f4274e6574b..80fe5091e96 100644
--- a/lib/Sema/ConstraintSystem.cpp
+++ b/lib/Sema/ConstraintSystem.cpp
@@ -1488,6 +1488,11 @@ Type ConstraintSystem::getEffectiveOverloadType(const OverloadChoice &overload,
   if (isa<TypeDecl>(decl))
     return Type();
 
+  // Declarations returning unwrapped optionals don't have a single effective
+  // type.
+  if (decl->getAttrs().hasAttribute<ImplicitlyUnwrappedOptionalAttr>())
+    return Type();
+
   // Retrieve the interface type.
   auto type = decl->getInterfaceType();
   if (!type) {
diff --git a/test/Constraints/common_type.swift b/test/Constraints/common_type.swift
index 1a3d6a64919..bc96a135509 100644
--- a/test/Constraints/common_type.swift
+++ b/test/Constraints/common_type.swift
@@ -7,6 +7,9 @@ struct X {
 
   subscript(_: Int) -> String { return "" }
   subscript(_: Double) -> String { return "" }
+
+  func iuo(_: Int) -> Int! { return 0 }
+  func iuo(_: Double) -> Int! { return 0 }
 }
 
 struct Y {
@@ -15,6 +18,9 @@ struct Y {
 
   subscript(_: Int) -> Substring { return "" }
   subscript(_: Double) -> Substring { return "" }
+
+  func iuo(_: Int) -> Double! { return 0 }
+  func iuo(_: Double) -> Double! { return 0 }
 }
 
 func f(_: Int) -> X { return X() }
@@ -29,11 +35,16 @@ func testCallCommonType() {
 }
 
 func testSubscriptCommonType() {
-  // FIXME: This will work once we have more filtering of subscripts.
   // CHECK: subscript_expr
   // CHECK: overload set choice binding $T{{[0-9]+}} := (Int) -> X
-  // CHECK-NOT: (common result type for $T{{[0-9]+}} is String)
+  // CHECK: (common result type for $T{{[0-9]+}} is String)
   // CHECK: (overload set choice binding $T{{[0-9]+}} := (Double) -> Y)
-  // CHECK-NOT: (common result type for $T{{[0-9]+}} is Substring)
+  // CHECK: (common result type for $T{{[0-9]+}} is Substring)
   _ = f(0)[0]
 }
+
+func testCommonTypeIUO() {
+  // CHECK: overload set choice binding $T{{[0-9]+}} := (Int) -> X
+  // CHECK-NOT: common result type
+    _ = f(0).iuo(0)
+}