Test static lambda hoisting works via LambdaDeserializer

LambdaMetafactory returns a ConstantCallSite bound to a shared instance
of a lambda, rather than a reference to the no-arg constructor. This is a
technique to avoid unnecessary allocations.

This test checks that we preserve this property when deserializing.

Author: retronym

src/test/java/scala/compat/java8/runtime/LambdaDeserializerTest.java

@@ -69,6 +69,18 @@ public void cached() {
         Assert.assertEquals(reconstituted1.getClass(), reconstituted2.getClass());
     }
 
+    @Test
+    public void cachedStatic() {
+        HashMap<String, MethodHandle> cache = new HashMap<>();
+        F1<Boolean, String> f1 = lambdaHost.lambdaBackedByStaticImplMethod();
+        // Check that deserialization of a static lambda always returns the
+        // same instance.
+        Assert.assertSame(reconstitute(f1, cache), reconstitute(f1, cache));
+
+        // (as is the case with regular invocation.)
+        Assert.assertSame(f1, lambdaHost.lambdaBackedByStaticImplMethod());
+    }
+
     @Test
     public void implMethodNameChanged() {
         F1<Boolean, String> f1 = lambdaHost.lambdaBackedByStaticImplMethod();