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[clang-tidy] Avoid expensive AST traversal in RedundantTypenameCheck #170540

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[clang-tidy] Avoid expensive AST traversal in RedundantTypenameCheck #170540

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10 changes: 7 additions & 3 deletions clang-tools-extra/clang-tidy/readability/RedundantTypenameCheck.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18,9 +18,13 @@ using namespace clang::ast_matchers;
namespace clang::tidy::readability {

void RedundantTypenameCheck::registerMatchers(MatchFinder *Finder) {
Finder->addMatcher(typeLoc(unless(hasAncestor(decl(isInstantiated()))))
.bind("nonDependentTypeLoc"),
this);
Finder->addMatcher(
typeLoc(loc(TypeMatcher(anyOf(typedefType(), tagType(),
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@zwuis zwuis Dec 4, 2025

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Can we remove TypeMatcher?

-typeLoc(loc(TypeMatcher(anyOf(...))))
+typeLoc(loc(anyOf(...)))

deducedTemplateSpecializationType(),
templateSpecializationType()))),
unless(hasAncestor(decl(isInstantiated()))))
.bind("nonDependentTypeLoc"),
this);
Comment on lines +21 to +27
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@5chmidti 5chmidti Dec 3, 2025

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Can't we skip the hasAncestor part (I'd keep the anyOf you added anyways since we are producing less matches that way, which also improves perf)

typeLoc(loc(qualType(unless(dependentNameType())))

?

Then we are not checking a type that is dependent, which eliminates the need for the hasAncestor and also the

      if (NonDependentTypeLoc->getType()->isDependentType())
        return SourceLocation();

later

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@zwuis zwuis Dec 4, 2025 •
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It seems that we can remove unless(...). The traverse mode of this checker is TK_IgnoreUnlessSpelledInSource, so decl(isInstantiated()) doesn't matches anything IIUC.

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@ilovepi ilovepi Dec 5, 2025

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Those are great suggestions. Thanks 😃

I’ll try to clean those up and see how our test case works after.

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@localspook localspook Dec 5, 2025 •
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It seems that we can remove unless(...). The traverse mode of this checker is TK_IgnoreUnlessSpelledInSource, so decl(isInstantiated()) doesn't matches anything IIUC.

I originally added the unless to make this test case pass:

llvm-project/clang-tools-extra/test/clang-tidy/checkers/readability/redundant-typename.cpp

Lines 167 to 170 in 5911754

template <typename T>
void n(typename T::R *) {}
template void n<NotDependent>(NotDependent::R *);

Without the unless, we get a false positive; the traversal seems to descend into the instantiation despite the traversal mode being TK_IgnoreUnlessSpelledInSource. Experimenting just now, I tried setting the traversal mode on the matcher explicitly:

  Finder->addMatcher(traverse(TK_IgnoreUnlessSpelledInSource, typeLoc()
                         .bind("nonDependentTypeLoc")),
                     this);

And now it does work! Is there maybe an issue causing the traversal mode to not get propagated?

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@localspook localspook Dec 5, 2025 •
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Ah, bingo. There are a bunch of addMatcher overloads. The ones for DeclarationMatcher and StatementMatcher respect getCheckTraversalKind:

llvm-project/clang/lib/ASTMatchers/ASTMatchFinder.cpp

Lines 1670 to 1680 in 7220268

void MatchFinder::addMatcher(const DeclarationMatcher &NodeMatch,
MatchCallback *Action) {
std::optional<TraversalKind> TK;
if (Action)
TK = Action->getCheckTraversalKind();
if (TK)
Matchers.DeclOrStmt.emplace_back(traverse(*TK, NodeMatch), Action);
else
Matchers.DeclOrStmt.emplace_back(NodeMatch, Action);
Matchers.AllCallbacks.insert(Action);
}

And all the rest (including the one we're calling) just... don't?!

llvm-project/clang/lib/ASTMatchers/ASTMatchFinder.cpp

Lines 1712 to 1716 in 7220268

void MatchFinder::addMatcher(const TypeLocMatcher &NodeMatch,
MatchCallback *Action) {
Matchers.TypeLoc.emplace_back(NodeMatch, Action);
Matchers.AllCallbacks.insert(Action);
}


if (!getLangOpts().CPlusPlus20)
return;
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