Merge pull request #22938 from github/copilot/unused-static-side-effect-test

Respect initializer side effects in unused-variable queries

Author: jketema

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

@@ -46,6 +46,8 @@ predicate declarationHasSideEffects(Variable v) {
   )
   or
   v.getType() instanceof TemplateDependentType // may have a constructor/destructor
+  or
+  v.getInitializer().getExpr().mayBeImpure()
 }
 
 from LocalVariable v, Function f

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

@@ -15,6 +15,8 @@ import cpp
 
 predicate declarationHasSideEffects(Variable v) {
   exists(Class c | c = v.getUnspecifiedType() | c.hasConstructor() or c.hasDestructor())
+  or
+  v.getInitializer().getExpr().mayBeImpure()
 }
 
 from Variable v

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

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/unused-local-variable` and `cpp/unused-static-variable` queries no longer report variables whose initializers may have side effects.

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

@@ -4,7 +4,6 @@
 | code2.cpp:26:16:26:17 | v1 | Variable v1 is not used. |
 | code2.cpp:42:11:42:16 | myVar1 | Variable myVar1 is not used. |
 | code2.cpp:64:7:64:8 | v3 | Variable v3 is not used. |
-| code2.cpp:108:11:108:12 | v2 | Variable v2 is not used. |
 | code2.cpp:128:9:128:9 | b | Variable b is not used. |
 | code.c:10:18:10:18 | y | Variable y is not used. |
 | code.c:11:18:11:18 | z | Variable z is not used. |

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

@@ -105,7 +105,7 @@ void test_range_based_for()
 		output(v1);
 	}
 
-	for (int v2 : myContainer) // $ Alert // BAD: v2 is not used
+	for (int v2 : myContainer) // GOOD: iterator dereference may have side effects
 	{
 	}
 }
@@ -181,3 +181,15 @@ void test_maybe_unused()
 {
 	[[maybe_unused]] int unused; // GOOD: explicitly marked as maybe_unused
 }
+
+int globalVar;
+
+int initialize()
+{
+	return ++globalVar;
+}
+
+void test_initializer_side_effect()
+{
+	int initialized = initialize(); // GOOD (initializer has a side effect)
+}

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

@@ -29,3 +29,23 @@ class MyTemplateClass // never instantiated
 		// ...
 	}
 };
+
+int initialize()
+{
+	return ++globalVar;
+}
+
+static int initialized = initialize(); // GOOD (initializer has a side effect)
+
+struct WithConstructor
+{
+	WithConstructor() { ++globalVar; }
+};
+
+struct WithDestructor
+{
+	~WithDestructor() { ++globalVar; }
+};
+
+static WithConstructor constructed; // GOOD (constructor has a side effect)
+static WithDestructor destructed; // GOOD (destructor has a side effect)