Merge pull request #22994 from riemenschneider/java/unread-local-unnamed-variables

Java: Exclude unnamed variables from `java/local-variable-is-never-read`

Author: aschackmull

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

@@ -32,5 +32,7 @@ where
     f.getVariable().getVariable() = v and
     f.getExpr().getType().(RefType).hasQualifiedName("kotlin.ranges", ["IntRange", "LongRange"])
   ) and
+  // Unnamed variables (`_`) cannot be read.
+  not v.getDeclExpr().isAnonymous() and
   not readImplicitly(v)
 select v, "Variable '" + v + "' is never read."

java/ql/src/change-notes/2026-11-03-unread-local-unnamed-variables.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `java/local-variable-is-never-read` query no longer reports unnamed variables (`_`), for example in `case Foo _ ->` patterns, `for (var _ : items)` loops, and `var _ = ...;` declarations.

java/ql/test/query-tests/UnreadLocal/UnreadLocal/UnnamedVariables.java

@@ -0,0 +1,45 @@
+package test;
+
+import java.util.List;
+
+public class UnnamedVariables
+{
+  sealed interface Shape permits Circle, Square {}
+  record Circle(double radius) implements Shape {}
+  record Square(double side) implements Shape {}
+  record Pair(Object left, Object right) {}
+
+  // Not flagged: unnamed variables cannot be read
+  static int side(Shape shape)
+  {
+    return switch (shape) {
+      case Circle _ -> 0;
+      case Square s -> (int) s.side();
+    };
+  }
+
+  static Object left(Object o)
+  {
+    if (o instanceof Pair(var left, _)) {
+      return left;
+    }
+    return switch (o) {
+      case Pair(Object left, Object _) -> left;
+      default -> null;
+    };
+  }
+
+  static int count(List<String> items)
+  {
+    int n = 0;
+    for (String _ : items) {
+      n++;
+    }
+    return n;
+  }
+
+  static void removeFirst(List<String> items)
+  {
+    var _ = items.remove(0);
+  }
+}

java/ql/test/query-tests/UnreadLocal/options

@@ -0,0 +1 @@
+//semmle-extractor-options: --javac-args --release 22