Fix false positives in java/unused parameter

Methods that are mentioned in a member reference expression should count
as rootdefs for the unused parameter query. Such methods have to match
the functional interface of the reference expression, so it is to be
expected that they will sometimes have to declare parameters that they
don't actually use.

java/ql/src/semmle/code/java/deadcode/DeadCode.qll

@@ -292,6 +292,10 @@ class RootdefCallable extends Callable {
     // a body that also doesn't.
     not hasUsefulBody(this) and
     not exists(Method m | hasUsefulBody(m) | m.overridesOrInstantiates+(this))
+    or
+    // Methods that are the target of a member reference need to implement
+    // the exact signature of the resulting functional interface.
+    exists(MemberRefExpr mre | mre.getReferencedCallable() = this)
   }
 }
 

java/ql/test/query-tests/dead-code/UselessParameter/Test.java

@@ -40,6 +40,10 @@ abstract class C implements I {
 	}
 }
 
+interface F {
+	void doSomething(int arg2);
+}
+
 public class Test {
 	// OK: external interface
 	public static void main(String[] args) {}
@@ -54,6 +58,13 @@ public class Test {
 	public static void foo(Object bar) {
 		throw new UnsupportedOperationException();
 	}
+
+	public static F getF() {
+		return Test::myFImplementation;
+	}
+
+	// OK: mentioned in member reference
+	private static void myFImplementation(int foo) {}
 	
 	// OK: native method
 	native int baz(int x);