Merge pull request #22932 from github/copilot/issue-22928-test-cases

Respect maybe_unused in unused-variable queries

Author: jketema

cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql

@@ -57,5 +57,6 @@ where
   not declarationHasSideEffects(v) and
   not exists(AsmStmt s | f = s.getEnclosingFunction()) and
   not v.getAnAttribute().getName() = "unused" and
+  not v.getAnAttribute().getName() = "maybe_unused" and
   not f.hasErrors() // Unextracted expressions may use `v`
 select v, "Variable " + v.getName() + " is not used."

cpp/ql/src/Best Practices/Unused Entities/UnusedStaticVariables.ql

@@ -26,5 +26,6 @@ where
   not v instanceof MemberVariable and
   not declarationHasSideEffects(v) and
   not v.getAnAttribute().hasName("used") and
-  not v.getAnAttribute().hasName("unused")
+  not v.getAnAttribute().hasName("unused") and
+  not v.getAnAttribute().hasName("maybe_unused")
 select v, "Static variable " + v.getName() + " is never read."

cpp/ql/src/change-notes/2026-10-29-unused-maybe-unused.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+The `cpp/unused-local-variable` and `cpp/unused-static-variable` queries no longer report variables marked with the C++17 `[[maybe_unused]]` attribute.

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp

@@ -176,3 +176,8 @@ void test_switch_initializer()
 		// ...
 	}
 }
+
+void test_maybe_unused()
+{
+	[[maybe_unused]] int unused; // GOOD: explicitly marked as maybe_unused
+}

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -1,4 +1,4 @@
-
+// semmle-extractor-options: -std=c++17
 int globalVar; // GOOD (not static)
 static int staticVar1; // GOOD (used)
 static int staticVar2; // GOOD (used)
@@ -10,6 +10,7 @@ static __attribute__((__unused__)) int staticVar7; // GOOD (unused but this is e
 const int constVar8 = 8; // $ Alert // BAD (const defaults to static)
 extern const int constVar9 = 9; // GOOD
 static int staticVar10 = 10; // $ SPURIOUS: Alert // GOOD [FALSE POSITIVE] (referenced in a never instantiated template)
+[[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as maybe_unused)
 
 void f()
 {