Treat Void formal arguments as @Nullable

nullaway/src/main/java/com/uber/nullaway/NullAway.java

@@ -1445,6 +1445,16 @@ private Description handleInvocation(
           } else {
             continue;
           }
+        } else if (ASTHelpers.isSameType(
+            param.type, Suppliers.JAVA_LANG_VOID_TYPE.get(state), state)) {
+          // Temporarily treat a Void argument type as if it were @Nullable Void. Handling of Void
+          // without
+          // special-casing, as recommeded by JSpecify might: a) require generics support and, b)
+          // require
+          // checking that third-party libraries considered annotated adopt JSpecify semantics.
+          // See the suppression in https://github.com/uber/NullAway/pull/608 for an example of why
+          // this is needed.
+          continue;
         }
         // we need to call paramHasNullableAnnotation here since the invoked method may be defined
         // in a class file

nullaway/src/main/java/com/uber/nullaway/dataflow/cfg/NullAwayCFGBuilder.java

@@ -113,14 +113,12 @@ public NullAwayCFGTranslationPhaseOne(
       this.handler = handler;
     }
 
-    @SuppressWarnings("NullAway") // (Void)null issue
-    private void scanWithVoid(Tree tree) {
-      this.scan(tree, (Void) null);
-    }
-
     /**
      * Obtain the type mirror for a given class, used for exception throwing.
      *
+     * <p>We use this method to expose the otherwise protected method {@link #getTypeMirror(Class)}
+     * to handlers.
+     *
      * @param klass a Java class
      * @return the corresponding type mirror
      */
@@ -144,7 +142,7 @@ public void insertThrowOnFalse(Node booleanExpressionNode, TypeMirror errorType)
       Preconditions.checkNotNull(booleanExpressionTree);
       Label falsePreconditionEntry = new Label();
       Label endPrecondition = new Label();
-      this.scanWithVoid(booleanExpressionTree);
+      this.scan(booleanExpressionTree, (Void) null);
       ConditionalJump cjump = new ConditionalJump(endPrecondition, falsePreconditionEntry);
       this.extendWithExtendedNode(cjump);
       this.addLabelForNextNode(falsePreconditionEntry);

nullaway/src/test/java/com/uber/nullaway/NullAwayCoreTests.java

@@ -623,6 +623,36 @@ public void nullableOnJavaLangVoid() {
         .doTest();
   }
 
+  @Test
+  public void nullableOnJavaLangVoidWithCast() {
+    defaultCompilationHelper
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import javax.annotation.Nullable;",
+            "class Test {",
+            "  // Currently unhandled. In fact, it *should* produce an error. This entire test case",
+            "  // needs to be rethought once we properly support generics, such that it works on T v",
+            "  // when T == @Nullable Void, but not when T == Void. Without generics, though, this is the",
+            "  // best we can do.",
+            "  @SuppressWarnings(\"NullAway\")",
+            "  private Void v = (Void)null;",
+            "  Void foo1() {",
+            "    // temporarily, we treat a Void return type as if it was @Nullable Void",
+            "    return (Void)null;",
+            "  }",
+            "  // Temporarily, we treat any Void formal as if it were @Nullable Void",
+            "  void consumeVoid(Void v) {",
+            "  }",
+            "  @Nullable Void foo2() {",
+            "    consumeVoid(null); // See comment on consumeVoid for why this is allowed",
+            "    consumeVoid((Void)null);",
+            "    return (Void)null;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
   @Test
   public void staticCallZeroArgsNullCheck() {
     defaultCompilationHelper