Only flag lambda types in FUNCTIONAL_INTERFACE_TYPES_TO_CHECK, but keep the underscore suggestion if it is.

core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java

@@ -248,13 +248,10 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
       if (!unusedElements.containsKey(unusedSymbol)) {
         isEverUsed.add(unusedSymbol);
       }
-      boolean suggestUnderscore =
-          suggestUnderscore(state, isEverUsed, unusedSymbol, specs, allUsageSites);
       SuggestedFix makeFirstAssignmentDeclaration =
           makeAssignmentDeclaration(unusedSymbol, specs, allUsageSites, state);
       // Don't complain if this is a public method and we only overwrote it once.
-      if ((onlyCheckForReassignments.contains(unusedSymbol) && specs.size() <= 1)
-          && !suggestUnderscore) {
+      if (onlyCheckForReassignments.contains(unusedSymbol) && specs.size() <= 1) {
         continue;
       }
       Tree unused = specs.iterator().next().assignmentPath().getLeaf();
@@ -267,7 +264,7 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
       } else {
         fixes.addAll(buildUnusedVarFixes(symbol, allUsageSites, state));
       }
-      if (suggestUnderscore) {
+      if (suggestUnderscore(state, isEverUsed, unusedSymbol, specs, allUsageSites)) {
         fixes.add(SuggestedFixes.renameVariable((VariableTree) unused, "_", state));
       }
       state.reportMatch(
@@ -657,8 +654,11 @@ private void handleVariable(VariableTree variableTree) {
       VarSymbol symbol = getSymbol(variableTree);
       var parent = getCurrentPath().getParentPath().getLeaf();
       if (parent instanceof LambdaExpressionTree) {
-        unusedElements.put(symbol, getCurrentPath());
-        usageSites.put(symbol, getCurrentPath());
+        if (FUNCTIONAL_INTERFACE_TYPES_TO_CHECK.stream()
+            .anyMatch(t -> isSubtype(getType(parent), state.getTypeFromString(t), state))) {
+          unusedElements.put(symbol, getCurrentPath());
+          usageSites.put(symbol, getCurrentPath());
+        }
         return;
       }
       if (symbol.getKind() == ElementKind.FIELD

core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java

@@ -1958,7 +1958,6 @@ void test() {
 
   @Test
   public void unusedFunctionalInterfaceParameter() {
-    assume().that(Runtime.version().feature()).isAtLeast(22);
     helper
         .addSourceLines(
             "Test.java",
@@ -2153,42 +2152,58 @@ public void suggestUnderscoreVariable() {
         .addInputLines(
             "Test.java",
             """
+            import java.util.Arrays;
+            import java.util.Collections;
             import java.util.function.Function;
 
             class Test {
-
-              static void sink(Object... o) {}
-
               public static void main(String[] args) {
                 int x;
                 var foo = new Object();
-                Function<String, Integer> f = s -> 1;
-                Function<String, Integer> g = (String s) -> 1;
-                Function<String, Integer> h = (s) -> 1;
-                sink(f, g, h);
+
+                Collections.sort(Arrays.asList(args), (a, b) -> a.isEmpty() ? 1 : 0);
               }
             }
             """)
         .addOutputLines(
             "Test.java",
             """
+            import java.util.Arrays;
+            import java.util.Collections;
             import java.util.function.Function;
 
             class Test {
+              public static void main(String[] args) {
+                var _ = new Object();
 
+                Collections.sort(Arrays.asList(args), (a, _) -> a.isEmpty() ? 1 : 0);
+              }
+            }
+            """)
+        .setFixChooser(Iterables::getLast)
+        .doTest();
+  }
+
+  @Test
+  public void lambdaParameter_noFinding() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.util.function.Function;
+
+            class Test {
               static void sink(Object... o) {}
 
               public static void main(String[] args) {
-
-                var _ = new Object();
-                Function<String, Integer> f = _ -> 1;
-                Function<String, Integer> g = (String _) -> 1;
-                Function<String, Integer> h = (_) -> 1;
+                Function<String, Integer> f = s -> 1;
+                Function<String, Integer> g = (String s) -> 1;
+                Function<String, Integer> h = (s) -> 1;
                 sink(f, g, h);
               }
             }
             """)
-        .setFixChooser(Iterables::getLast)
+        .expectUnchanged()
         .doTest(TEXT_MATCH);
   }
 }