Merge pull request github#10789 from tamasvajk/kotlin-useless-params

Kotlin: reduce FPs in useless parameter check for Kotlin code

Author: tamasvajk

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

@@ -304,6 +304,15 @@ class RootdefCallable extends Callable {
     this.getAnAnnotation() instanceof OverrideAnnotation
     or
     this.hasModifier("override")
+    or
+    // Exclude generated callables, such as `...$default` ones extracted from Kotlin code.
+    this.isCompilerGenerated()
+    or
+    // Exclude Kotlin serialization constructors.
+    this.(Constructor)
+        .getParameterType(this.getNumberOfParameters() - 1)
+        .(RefType)
+        .hasQualifiedName("kotlinx.serialization.internal", "SerializationConstructorMarker")
   }
 }
 

java/ql/test/kotlin/query-tests/UselessParameter/Test.kt

@@ -7,3 +7,5 @@ class B : A<B, Int> {
         println("a")
     }
 }
+
+fun fn(a: Int = 10) {}

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

@@ -0,0 +1 @@
+| Test.kt:11:8:11:18 | a | The parameter 'a' is never used. |