Support method references to sibling classes in the default package.

end-to-end-tests/src/test/java/DefaultPackageTest.java

@@ -0,0 +1,19 @@
+// Copyright © 2013-2016 Esko Luontola and other Retrolambda contributors
+// This software is released under the Apache License 2.0.
+// The license text is at http://www.apache.org/licenses/LICENSE-2.0
+
+import org.junit.Test;
+
+public final class DefaultPackageTest {
+    @Test
+    public void method_reference_to_sibling_class() {
+        SiblingClass sibling = new SiblingClass();
+        Runnable lambda = sibling::method;
+        lambda.run();
+    }
+}
+
+class SiblingClass {
+    void method() {
+    }
+}

retrolambda/src/main/java/net/orfjackal/retrolambda/lambdas/BackportLambdaInvocations.java

@@ -104,8 +104,12 @@ private boolean isOwnedMethodVisible(Handle implMethod) {
     }
 
     private boolean isNonOwnedMethodVisible(Handle implMethod) {
-        String classPackage = className.substring(0, className.lastIndexOf('/'));
-        String implPackage = implMethod.getOwner().substring(0, implMethod.getOwner().lastIndexOf('/'));
+        int classNameLastSlash = className.lastIndexOf('/');
+        String classPackage = classNameLastSlash == -1 ? "" : className.substring(0, classNameLastSlash);
+
+        int implMethodLastSlash = implMethod.getOwner().lastIndexOf('/');
+        String implPackage = implMethodLastSlash == -1 ? "" : implMethod.getOwner().substring(0, implMethodLastSlash);
+
         if (classPackage.equals(implPackage)) {
             return true; // All method visibilities in the same package will be visible.
         }