Code quality improvements + add dedicated DeadRefTypes test

java/ql/lib/config/semmlecode.dbscheme

@@ -1224,7 +1224,7 @@ ktPropertyDelegates(
 )
 
 compiler_generated(
-  unique int id: @top ref,
+  unique int id: @element ref,
   int kind: int ref
-  // 1: Kotlin declaring classes of adapter functions
-);
+  // 1: Declaring classes of adapter functions in Kotlin
+)

java/ql/lib/semmle/code/java/Element.qll

@@ -41,6 +41,9 @@ class Element extends @element, Top {
 
   /** Cast this element to a `Documentable`. */
   Documentable getDoc() { result = this }
+
+  /** Holds if this is an auxiliary program element generated by the compiler. */
+  predicate isCompilerGenerated() { compiler_generated(this, _) }
 }
 
 /**

java/ql/src/Violations of Best Practice/Dead Code/DeadRefTypes.ql

@@ -52,7 +52,7 @@ predicate dead(RefType dead) {
   // Insist all source ancestors are dead as well.
   forall(RefType t | t.fromSource() and t = getASuperTypePlus(dead) | dead(t)) and
   // Exclude compiler generated classes (e.g. declaring type of adapter functions in Kotlin)
-  not compiler_generated(dead, _)
+  not dead.isCompilerGenerated()
 }
 
 from RefType t, string kind

java/ql/test/kotlin/library-tests/reflection/reflection.ql

@@ -87,4 +87,4 @@ query predicate modifiers(ClassInstanceExpr e, Method m, string modifier) {
   m.hasModifier(modifier)
 }
 
-query predicate compGenerated(Top t, int i) { compiler_generated(t, i) }
+query predicate compGenerated(Element e, int i) { compiler_generated(e, i) }

java/ql/test/kotlin/query-tests/DeadRefTypes/DeadRefTypes.expected

@@ -0,0 +1 @@
+| test.kt:1:1:1:20 | C1 | Unused class: C1 is not referenced within this codebase. If not used as an external API it should be removed. |

java/ql/test/kotlin/query-tests/DeadRefTypes/DeadRefTypes.qlref

@@ -0,0 +1 @@
+Violations of Best Practice/Dead Code/DeadRefTypes.ql

java/ql/test/kotlin/query-tests/DeadRefTypes/test.kt

@@ -0,0 +1,15 @@
+private class C1 { }
+
+private class C2 { }
+
+fun fn() {
+    val c = C2()
+}
+
+fun fn1() = 5
+
+fun fn2(f: () -> Unit) = f()
+
+fun adapted() {
+    fn2(::fn1)
+}