From 6bd568d418694043bfec7961dff0ba5559bb856a Mon Sep 17 00:00:00 2001
From: Liam Miller-Cushon <cushon@google.com>
Date: Tue, 16 Jul 2024 10:52:31 -0700
Subject: [PATCH] Don't complain about unused variables named `_`

See https://openjdk.org/jeps/456

Fixes https://github.com/google/error-prone/issues/4451

PiperOrigin-RevId: 652904059
---
 .../google/errorprone/util/RuntimeVersion.java    |  5 +++++
 .../errorprone/bugpatterns/UnusedVariable.java    |  1 +
 .../bugpatterns/UnusedVariableTest.java           | 15 +++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/check_api/src/main/java/com/google/errorprone/util/RuntimeVersion.java b/check_api/src/main/java/com/google/errorprone/util/RuntimeVersion.java
index 14ae3dd37fc..0785489c645 100644
--- a/check_api/src/main/java/com/google/errorprone/util/RuntimeVersion.java
+++ b/check_api/src/main/java/com/google/errorprone/util/RuntimeVersion.java
@@ -79,6 +79,11 @@ public static boolean isAtLeast21() {
     return FEATURE >= 21;
   }
 
+  /** Returns true if the current runtime is JDK 22 or newer. */
+  public static boolean isAtLeast22() {
+    return FEATURE >= 22;
+  }
+
   /**
    * Returns the latest {@code --release} version.
    *
diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java b/core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java
index 71e12a7f237..0e8f41a500e 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java
@@ -198,6 +198,7 @@ public final class UnusedVariable extends BugChecker implements CompilationUnitT
     this.exemptNames =
         ImmutableSet.<String>builder()
             .add("ignored")
+            .add("") // `var _ = ...` is handled as an empty variable name
             .addAll(flags.getListOrEmpty("Unused:exemptNames"))
             .build();
 
diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java
index 92107250de7..4db2f04b54f 100644
--- a/core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java
+++ b/core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java
@@ -1679,4 +1679,19 @@ public void parameterUsedInOverride() {
         .expectUnchanged()
         .doTest();
   }
+
+  @Test
+  public void underscoreVariable() {
+    assumeTrue(RuntimeVersion.isAtLeast22());
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  public static void main(String[] args) {",
+            "    var _ = new Object();",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
 }