diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index 8c766b12ffc9d..d4713caa7db47 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -4114,8 +4114,10 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
       if (auto *storage = dyn_cast<AbstractStorageDecl>(decl)) {
         // If this is an attempt to access read-only member via
         // writable key path, let's fail this choice early.
+        auto &ctx = getASTContext();
         if (isReadOnlyKeyPathComponent(storage) &&
-            keyPath == getASTContext().getWritableKeyPathDecl()) {
+            (keyPath == ctx.getWritableKeyPathDecl() ||
+             keyPath == ctx.getReferenceWritableKeyPathDecl())) {
           result.addUnviable(
               candidate,
               MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember);
@@ -4126,7 +4128,7 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
         // on the other hand if setter is mutating there is no point
         // of attempting `ReferenceWritableKeyPath` overload.
         if (storage->isSetterMutating() &&
-            keyPath == getASTContext().getReferenceWritableKeyPathDecl()) {
+            keyPath == ctx.getReferenceWritableKeyPathDecl()) {
           result.addUnviable(
               candidate,
               MemberLookupResult::UR_ReferenceWritableKeyPathOnMutatingMember);
diff --git a/test/Constraints/keypath_dynamic_member_lookup.swift b/test/Constraints/keypath_dynamic_member_lookup.swift
index fb4f1bb899870..80ec2c3ade99a 100644
--- a/test/Constraints/keypath_dynamic_member_lookup.swift
+++ b/test/Constraints/keypath_dynamic_member_lookup.swift
@@ -250,3 +250,38 @@ func test_recursive_dynamic_lookup(_ lens: Lens<Lens<Point>>) {
   // CHECK-NEXT: keypath $KeyPath<Lens<Lens<Rectangle>>, Lens<Lens<Int>>>, (root $Lens<Lens<Rectangle>>; settable_property $Lens<Lens<Point>>,  id @$s29keypath_dynamic_member_lookup4LensV0B6MemberACyqd__Gs15WritableKeyPathCyxqd__G_tcluig : {{.*}})
   _ = \Lens<Lens<Rectangle>>.topLeft.x
 }
+
+@dynamicMemberLookup
+struct RefWritableBox<T> {
+  var obj: T
+
+  init(_ obj: T) {
+    self.obj = obj
+  }
+
+  subscript<U>(dynamicMember member: KeyPath<T, U>) -> U {
+    get { return obj[keyPath: member] }
+  }
+
+  subscript<U>(dynamicMember member: ReferenceWritableKeyPath<T, U>) -> U {
+    get { return obj[keyPath: member] }
+    set { obj[keyPath: member] = newValue }
+  }
+}
+
+func prefer_readonly_keypath_over_reference_writable() {
+  class C {
+    let foo: Int
+
+    init(_ foo: Int) {
+      self.foo = foo
+    }
+  }
+
+  var box = RefWritableBox(C(42))
+  // expected-warning@-1 {{variable 'box' was never mutated; consider changing to 'let' constant}}
+
+  // CHECK: function_ref RefWritableBox.subscript.getter
+  // CHECK-NEXT: function_ref @$s29keypath_dynamic_member_lookup14RefWritableBoxV0B6Memberqd__s7KeyPathCyxqd__G_tcluig
+  _ = box.foo
+}