[clang-tools-extra] [clang-tidy] Fix FP/FN in cppcoreguidelines-missing-std-forward (PR #178651)
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cfe-commits at lists.llvm.org
Thu Feb 19 09:00:57 PST 2026
https://github.com/zeyi2 updated https://github.com/llvm/llvm-project/pull/178651
>From 4bf9b5fa2be82ad2ce454a86bfae26cc3747e4d5 Mon Sep 17 00:00:00 2001
From: mtx <mitchell.xu2 at gmail.com>
Date: Wed, 28 Jan 2026 23:39:33 +0800
Subject: [PATCH 1/3] [clang-tidy] Fix FP/FN in
cppcoreguidelines-missing-std-forward
---
.../MissingStdForwardCheck.cpp | 20 ++++----
clang-tools-extra/docs/ReleaseNotes.rst | 8 ++++
.../cppcoreguidelines/missing-std-forward.cpp | 46 +++++++++++++++++++
3 files changed, 65 insertions(+), 9 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
index d1d81d510c8fb..357a5e46df823 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
@@ -106,20 +106,22 @@ void MissingStdForwardCheck::registerMatchers(MatchFinder *Finder) {
varDecl(hasInitializer(ignoringParenImpCasts(equalsBoundNode("call"))))));
auto CapturedInLambda = hasDeclContext(cxxRecordDecl(
- isLambda(),
- hasParent(lambdaExpr(forCallable(equalsBoundNode("func")),
- anyOf(CapturedInCaptureList, CapturedInBody)))));
+ isLambda(), hasParent(lambdaExpr(
+ anyOf(CapturedInCaptureList, CapturedInBody),
+ hasAncestor(functionDecl(equalsBoundNode("func")))))));
auto ToParam = hasAnyParameter(parmVarDecl(equalsBoundNode("param")));
auto ForwardCallMatcher = callExpr(
callExpr().bind("call"), argumentCountIs(1),
- hasArgument(0, declRefExpr(to(varDecl().bind("var")))),
- forCallable(
- anyOf(allOf(equalsBoundNode("func"),
- functionDecl(hasAnyParameter(parmVarDecl(allOf(
- equalsBoundNode("param"), equalsBoundNode("var")))))),
- CapturedInLambda)),
+ hasArgument(
+ 0, declRefExpr(to(
+ varDecl(anyOf(equalsBoundNode("param"),
+ hasSameNameAsBoundNode("param"),
+ hasInitializer(ignoringParenImpCasts(
+ declRefExpr(to(equalsBoundNode("param")))))))
+ .bind("var")))),
+ forCallable(anyOf(equalsBoundNode("func"), CapturedInLambda)),
callee(unresolvedLookupExpr(hasAnyDeclaration(
namedDecl(hasUnderlyingDecl(hasName(ForwardFunction)))))),
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 754880bd1a381..6faf18fc96b55 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -136,6 +136,14 @@ Changes in existing checks
the invalidating function in the warning message when a custom invalidation
function is used (via the `InvalidationFunctions` option).
+- Improved :doc:`cppcoreguidelines-missing-std-forward
+ <clang-tidy/checks/cppcoreguidelines/missing-std-forward>` check by:
+
+ - Correctly handling forwarding in deeply nested lambdas.
+
+ - Fixing a false negative when multiple parameters are used in a lambda and
+ only some of them are forwarded.
+
- Improved :doc:`llvm-use-ranges
<clang-tidy/checks/llvm/use-ranges>` check by adding support for the following
algorithms: ``std::accumulate``, ``std::replace_copy``, and
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
index 98c592db7ce22..47e2977d6fc12 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
@@ -105,6 +105,26 @@ void foo(X &&x, Y &&y) {
use(y);
}
+template <typename T>
+void nested_but_no_forward(T &&arg) {
+ // CHECK-MESSAGES: :[[@LINE-1]]:32: warning: forwarding reference parameter 'arg' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+ [&]()
+ {
+ [&]()
+ { consumes_all(arg); }();
+ }();
+}
+
+template <typename T, typename U>
+void nested_forward_only_one(T &&arg1, U &&arg2) {
+ // CHECK-MESSAGES: :[[@LINE-1]]:44: warning: forwarding reference parameter 'arg2' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+ [&]()
+ {
+ [&]()
+ { consumes_all(std::forward<T>(arg1)); }();
+ }();
+}
+
} // namespace positive_cases
namespace negative_cases {
@@ -182,6 +202,32 @@ void lambda_value_reference_auxiliary_var(T&& t) {
[&x = t]() { T other = std::forward<T>(x); };
}
+template <typename T>
+void nested_forward(T &&arg) {
+ [&]()
+ {
+ [&]()
+ { consumes_all(std::forward<T>(arg)); }();
+ }();
+}
+
+template <typename T>
+void triple_nested_forward(T &&arg) {
+ [&]()
+ {
+ [&]()
+ {
+ [&]()
+ { consumes_all(std::forward<T>(arg)); }();
+ }();
+ }();
+}
+
+template <class T>
+void lambda_renamed_capture(T&& t) {
+ [&a = t]() { consumes_all(std::forward<T>(a)); };
+}
+
} // namespace negative_cases
namespace deleted_functions {
>From 9a8af31a0d8563e9b89ffeccc08485d68fc5eea7 Mon Sep 17 00:00:00 2001
From: mtx <mitchell.xu2 at gmail.com>
Date: Thu, 12 Feb 2026 00:59:43 +0800
Subject: [PATCH 2/3] ~
---
.../checkers/cppcoreguidelines/missing-std-forward.cpp | 5 -----
1 file changed, 5 deletions(-)
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
index 47e2977d6fc12..e5f3421cb6b46 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
@@ -223,11 +223,6 @@ void triple_nested_forward(T &&arg) {
}();
}
-template <class T>
-void lambda_renamed_capture(T&& t) {
- [&a = t]() { consumes_all(std::forward<T>(a)); };
-}
-
} // namespace negative_cases
namespace deleted_functions {
>From 3408527d2b6421c174fe8dbff0dc4ab89ebe7ee9 Mon Sep 17 00:00:00 2001
From: mtx <mitchell.xu2 at gmail.com>
Date: Fri, 20 Feb 2026 00:55:55 +0800
Subject: [PATCH 3/3] hopefully better..?
---
.../MissingStdForwardCheck.cpp | 62 ++++++++++++-------
.../cppcoreguidelines/missing-std-forward.cpp | 28 +++++++++
2 files changed, 66 insertions(+), 24 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
index 357a5e46df823..0ffcbc9d4cccd 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
@@ -85,21 +85,41 @@ AST_MATCHER(VarDecl, hasIdentifier) {
return ID != nullptr && !ID->isPlaceholder();
}
+AST_MATCHER_P(ValueDecl, refersToBoundParm, std::string, ParamID) {
+ return Builder->removeBindings(
+ [&](const ast_matchers::internal::BoundNodesMap &Nodes) {
+ const auto *Param = Nodes.getNodeAs<ParmVarDecl>(ParamID);
+ if (!Param)
+ return true;
+
+ for (const ValueDecl *V = &Node; V;) {
+ if (V == Param)
+ return false;
+
+ const auto *VD = dyn_cast<VarDecl>(V);
+ const Expr *Init = (VD && VD->getType()->isReferenceType())
+ ? VD->getInit()
+ : nullptr;
+ const auto *DRE =
+ Init ? dyn_cast<DeclRefExpr>(Init->IgnoreParenImpCasts())
+ : nullptr;
+ V = DRE ? DRE->getDecl() : nullptr;
+ }
+ return true;
+ });
+}
+
} // namespace
void MissingStdForwardCheck::registerMatchers(MatchFinder *Finder) {
- auto RefToParmImplicit = allOf(
- equalsBoundNode("var"), hasInitializer(ignoringParenImpCasts(
- declRefExpr(to(equalsBoundNode("param"))))));
- auto RefToParm = capturesVar(
- varDecl(anyOf(hasSameNameAsBoundNode("param"), RefToParmImplicit)));
+ auto CapturedVar = varDecl(
+ anyOf(refersToBoundParm("param"), hasSameNameAsBoundNode("param")));
auto CaptureInRef =
allOf(hasCaptureDefaultKind(LambdaCaptureDefault::LCD_ByRef),
- unless(hasAnyCapture(
- capturesVar(varDecl(hasSameNameAsBoundNode("param"))))));
- auto CaptureByRefExplicit = hasAnyCapture(
- allOf(hasCaptureKind(LambdaCaptureKind::LCK_ByRef), RefToParm));
+ unless(hasAnyCapture(capturesVar(CapturedVar))));
+ auto CaptureByRefExplicit = hasAnyCapture(allOf(
+ hasCaptureKind(LambdaCaptureKind::LCK_ByRef), capturesVar(CapturedVar)));
auto CapturedInBody = lambdaExpr(anyOf(CaptureInRef, CaptureByRefExplicit));
auto CapturedInCaptureList = hasAnyCapture(capturesVar(
@@ -112,21 +132,15 @@ void MissingStdForwardCheck::registerMatchers(MatchFinder *Finder) {
auto ToParam = hasAnyParameter(parmVarDecl(equalsBoundNode("param")));
- auto ForwardCallMatcher = callExpr(
- callExpr().bind("call"), argumentCountIs(1),
- hasArgument(
- 0, declRefExpr(to(
- varDecl(anyOf(equalsBoundNode("param"),
- hasSameNameAsBoundNode("param"),
- hasInitializer(ignoringParenImpCasts(
- declRefExpr(to(equalsBoundNode("param")))))))
- .bind("var")))),
- forCallable(anyOf(equalsBoundNode("func"), CapturedInLambda)),
- callee(unresolvedLookupExpr(hasAnyDeclaration(
- namedDecl(hasUnderlyingDecl(hasName(ForwardFunction)))))),
-
- unless(anyOf(hasAncestor(typeLoc()),
- hasAncestor(expr(hasUnevaluatedContext())))));
+ auto ForwardCallMatcher =
+ callExpr(callExpr().bind("call"), argumentCountIs(1),
+ hasArgument(0, declRefExpr(to(CapturedVar)).bind("var")),
+ forCallable(anyOf(equalsBoundNode("func"), CapturedInLambda)),
+ callee(unresolvedLookupExpr(hasAnyDeclaration(
+ namedDecl(hasUnderlyingDecl(hasName(ForwardFunction)))))),
+
+ unless(anyOf(hasAncestor(typeLoc()),
+ hasAncestor(expr(hasUnevaluatedContext())))));
Finder->addMatcher(
parmVarDecl(
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
index e5f3421cb6b46..bbbd6fa1c9686 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
@@ -125,6 +125,16 @@ void nested_forward_only_one(T &&arg1, U &&arg2) {
}();
}
+template <typename T>
+void nested_rename_no_forward(T &&t) {
+ // CHECK-MESSAGES: :[[@LINE-1]]:35: warning: forwarding reference parameter 't' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+ [&x = t]() {
+ [&y = x]() {
+ (void)y;
+ }();
+ }();
+}
+
} // namespace positive_cases
namespace negative_cases {
@@ -202,6 +212,24 @@ void lambda_value_reference_auxiliary_var(T&& t) {
[&x = t]() { T other = std::forward<T>(x); };
}
+template <class T>
+void lambda_multi_level_rename(T &&t) {
+ [&x = t]() {
+ [&y = x]() {
+ T other = std::forward<T>(y);
+ }();
+ }();
+}
+
+template <class T>
+void lambda_implicit_and_rename(T &&t) {
+ [&]() {
+ [&y = t]() {
+ T other = std::forward<T>(y);
+ }();
+ }();
+}
+
template <typename T>
void nested_forward(T &&arg) {
[&]()
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