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Respect maybe_unused in unused-variable queries #22932

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Merged
jketema merged 3 commits into from
Oct 29, 2026
+14 −2
Merged

Respect maybe_unused in unused-variable queries #22932

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6e75b96
Add maybe_unused regression tests for unused-variable queries
jketema Oct 29, 2026
24b6e60
Respect maybe_unused in unused-variable queries
jketema Oct 29, 2026
7b47e9b
Document maybe_unused query improvement
jketema Oct 29, 2026
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1 change: 1 addition & 0 deletions cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql
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Expand Up @@ -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."
3 changes: 2 additions & 1 deletion cpp/ql/src/Best Practices/Unused Entities/UnusedStaticVariables.ql
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Expand Up @@ -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."
4 changes: 4 additions & 0 deletions cpp/ql/src/change-notes/2026-10-29-unused-maybe-unused.md
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@@ -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.
5 changes: 5 additions & 0 deletions cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp
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Expand Up @@ -176,3 +176,8 @@ void test_switch_initializer()
// ...
}
}

void test_maybe_unused()
{
[[maybe_unused]] int unused; // GOOD: explicitly marked as maybe_unused
}
3 changes: 2 additions & 1 deletion cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp
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@@ -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)
Expand All @@ -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()
{
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