[clang-tools-extra] [clang-tidy] Add bugprone-default-lambda-capture (PR #160150)
JJ Marr via cfe-commits
cfe-commits at lists.llvm.org
Mon Sep 22 09:48:11 PDT 2025
https://github.com/jjmarr-amd created https://github.com/llvm/llvm-project/pull/160150
This is a new linter check that implements Scott Meyers' recommendation to avoid default lambda captures, e.g. `[&]() { ... }`.
The check does not contain an autofix. This will be added in a future PR to reduce the code review burden on this PR.
This PR was generated with AI tools. My workflow involved 20 minutes of discussion about the check with AI, after which the AI generated commit `35df1c6`. I then reviewed the PR at https://github.com/jjmarr-amd/llvm-project/pull/1 and made various changes to the LLM output in follow-up commits.
I disagreed with the LLM on a few somewhat important points. First, the original warning called out the type of default capture (by-value or by-reference) in the warning message. I felt that was overengineered and unnecessary. Second, the LLM created an `AST_MATCHER` that matched every lambda, then checked for the presence of a default capture in `check()`. From previous code reviews I believe it's better to put that logic into the matcher itself.
>From 35df1c6ea3af8dbaea2d7049ae79b14ce39f0774 Mon Sep 17 00:00:00 2001
From: JJ Marr <JJ.Marr at amd.com>
Date: Fri, 11 Jul 2025 14:40:57 -0400
Subject: [PATCH 1/7] Bugprone-default-lambda-capture
on-behalf-of: @AMD <JJ.Marr at amd.com>
---
.../bugprone/BugproneTidyModule.cpp | 3 +
.../clang-tidy/bugprone/CMakeLists.txt | 1 +
.../bugprone/DefaultLambdaCaptureCheck.cpp | 35 +++++++
.../bugprone/DefaultLambdaCaptureCheck.h | 26 +++++
clang-tools-extra/docs/ReleaseNotes.rst | 9 ++
.../bugprone/default-lambda-capture.rst | 42 ++++++++
.../docs/clang-tidy/checks/list.rst | 1 +
.../bugprone/default-lambda-capture.cpp | 98 +++++++++++++++++++
8 files changed, 215 insertions(+)
create mode 100644 clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
create mode 100644 clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
create mode 100644 clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
diff --git a/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp b/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
index 491de6acea2b7..db99d57c511b8 100644
--- a/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
@@ -23,6 +23,7 @@
#include "CopyConstructorInitCheck.h"
#include "CrtpConstructorAccessibilityCheck.h"
#include "DanglingHandleCheck.h"
+#include "DefaultLambdaCaptureCheck.h"
#include "DynamicStaticInitializersCheck.h"
#include "EasilySwappableParametersCheck.h"
#include "EmptyCatchCheck.h"
@@ -134,6 +135,8 @@ class BugproneModule : public ClangTidyModule {
"bugprone-copy-constructor-init");
CheckFactories.registerCheck<DanglingHandleCheck>(
"bugprone-dangling-handle");
+ CheckFactories.registerCheck<DefaultLambdaCaptureCheck>(
+ "bugprone-default-lambda-capture");
CheckFactories.registerCheck<DynamicStaticInitializersCheck>(
"bugprone-dynamic-static-initializers");
CheckFactories.registerCheck<EasilySwappableParametersCheck>(
diff --git a/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt b/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
index 46bc8efd44bc5..66125c9d22c1c 100644
--- a/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
@@ -19,6 +19,7 @@ add_clang_library(clangTidyBugproneModule STATIC
CopyConstructorInitCheck.cpp
CrtpConstructorAccessibilityCheck.cpp
DanglingHandleCheck.cpp
+ DefaultLambdaCaptureCheck.cpp
DynamicStaticInitializersCheck.cpp
EasilySwappableParametersCheck.cpp
EmptyCatchCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
new file mode 100644
index 0000000000000..6c95fbc984c5a
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -0,0 +1,35 @@
+#include "DefaultLambdaCaptureCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+void DefaultLambdaCaptureCheck::registerMatchers(MatchFinder *Finder) {
+ // Match any lambda expression
+ Finder->addMatcher(lambdaExpr().bind("lambda"), this);
+}
+
+void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
+ const auto *Lambda = Result.Nodes.getNodeAs<LambdaExpr>("lambda");
+ if (!Lambda)
+ return;
+
+ // Check if lambda has a default capture
+ if (Lambda->getCaptureDefault() == LCD_None)
+ return;
+
+ SourceLocation DefaultCaptureLoc = Lambda->getCaptureDefaultLoc();
+ if (DefaultCaptureLoc.isInvalid())
+ return;
+
+ const char *CaptureKind =
+ (Lambda->getCaptureDefault() == LCD_ByCopy) ? "by-copy" : "by-reference";
+
+ diag(DefaultCaptureLoc, "lambda %0 default capture is discouraged; "
+ "prefer to capture specific variables explicitly")
+ << CaptureKind;
+}
+
+} // namespace clang::tidy::bugprone
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
new file mode 100644
index 0000000000000..ac47c866cfccd
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
@@ -0,0 +1,26 @@
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DEFAULT_LAMBDA_CAPTURE_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DEFAULT_LAMBDA_CAPTURE_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::bugprone {
+
+/** Flags lambdas that use default capture modes
+ *
+ * For the user-facing documentation see:
+ * http://clang.llvm.org/extra/clang-tidy/checks/bugprone/default-lambda-capture.html
+ */
+class DefaultLambdaCaptureCheck : public ClangTidyCheck {
+public:
+ DefaultLambdaCaptureCheck(StringRef Name, ClangTidyContext *Context)
+ : ClangTidyCheck(Name, Context) {}
+ void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+ void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+ std::optional<TraversalKind> getCheckTraversalKind() const override {
+ return TK_IgnoreUnlessSpelledInSource;
+ }
+};
+
+} // namespace clang::tidy::bugprone
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DEFAULT_LAMBDA_CAPTURE_H
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 7cdff86beeec6..30d0bffb78f39 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -194,6 +194,15 @@ New checks
Finds virtual function overrides with different visibility than the function
in the base class.
+- New :doc:`bugprone-default-lambda-capture
+ <clang-tidy/checks/bugprone/default-lambda-capture>` check.
+
+ Finds lambda expressions that use default capture modes (``[=]`` or ``[&]``)
+ and suggests using explicit captures instead. Default captures can lead to
+ subtle bugs including dangling references with ``[&]``, unnecessary copies
+ with ``[=]``, and make code less maintainable by hiding which variables are
+ actually being captured.
+
New check aliases
^^^^^^^^^^^^^^^^^
diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
new file mode 100644
index 0000000000000..026acf9f1894b
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
@@ -0,0 +1,42 @@
+.. title:: clang-tidy - bugprone-default-lambda-capture
+
+bugprone-default-lambda-capture
+===============================
+
+Warns when lambda expressions use default capture modes (``[=]`` or ``[&]``)
+instead of explicitly capturing specific variables. Default captures can make
+code less readable and more error-prone by making it unclear which variables
+are being captured and how.
+
+Implements Item 31 of Effective Modern C++ by Scott Meyers.
+
+Example
+-------
+
+.. code-block:: c++
+
+ void example() {
+ int x = 1;
+ int y = 2;
+
+ // Bad - default capture by copy
+ auto lambda1 = [=]() { return x + y; };
+
+ // Bad - default capture by reference
+ auto lambda2 = [&]() { return x + y; };
+
+ // Good - explicit captures
+ auto lambda3 = [x, y]() { return x + y; };
+ auto lambda4 = [&x, &y]() { return x + y; };
+ }
+
+The check will warn on:
+
+- Default capture by copy: ``[=]``
+- Default capture by reference: ``[&]``
+- Mixed captures with defaults: ``[=, &x]`` or ``[&, x]``
+
+The check will not warn on:
+
+- Explicit captures: ``[x]``, ``[&x]``, ``[x, y]``, ``[this]``
+- Empty capture lists: ``[]``
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index c490d2ece2e0a..cb6254ee36f42 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -91,6 +91,7 @@ Clang-Tidy Checks
:doc:`bugprone-copy-constructor-init <bugprone/copy-constructor-init>`, "Yes"
:doc:`bugprone-crtp-constructor-accessibility <bugprone/crtp-constructor-accessibility>`, "Yes"
:doc:`bugprone-dangling-handle <bugprone/dangling-handle>`,
+ :doc:`bugprone-default-lambda-capture <bugprone/default-lambda-capture>`,
:doc:`bugprone-dynamic-static-initializers <bugprone/dynamic-static-initializers>`,
:doc:`bugprone-easily-swappable-parameters <bugprone/easily-swappable-parameters>`,
:doc:`bugprone-empty-catch <bugprone/empty-catch>`,
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
new file mode 100644
index 0000000000000..f820b4c9b8a0e
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
@@ -0,0 +1,98 @@
+// RUN: %check_clang_tidy %s bugprone-default-lambda-capture %t
+
+void test_default_captures() {
+ int value = 42;
+ int another = 10;
+
+ auto lambda1 = [=](int x) { return value + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto lambda2 = [&](int x) { return value + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto lambda3 = [=, &another](int x) { return value + another + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto lambda4 = [&, value](int x) { return value + another + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+}
+
+void test_acceptable_captures() {
+ int value = 42;
+ int another = 10;
+
+ auto lambda1 = [value](int x) { return value + x; };
+ auto lambda2 = [&value](int x) { return value + x; };
+ auto lambda3 = [value, another](int x) { return value + another + x; };
+ auto lambda4 = [&value, &another](int x) { return value + another + x; };
+
+ auto lambda5 = [](int x, int y) { return x + y; };
+
+ struct S {
+ int member = 5;
+ void foo() {
+ auto lambda = [this]() { return member; };
+ }
+ };
+}
+
+void test_nested_lambdas() {
+ int outer_var = 1;
+ int middle_var = 2;
+ int inner_var = 3;
+
+ auto outer = [=]() {
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto inner = [&](int x) { return outer_var + middle_var + inner_var + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ return inner(10);
+ };
+}
+
+void test_lambda_returns() {
+ int a = 1, b = 2, c = 3;
+
+ auto create_adder = [=](int x) {
+ // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ return [x](int y) { return x + y; }; // Inner lambda is fine - explicit capture
+ };
+
+ auto func1 = [&]() { return a; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto func2 = [=]() { return b; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+}
+
+class TestClass {
+ int member = 42;
+
+public:
+ void test_member_function_lambdas() {
+ int local = 10;
+
+ auto lambda1 = [=]() { return member + local; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto lambda2 = [&]() { return member + local; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+
+ auto lambda3 = [this, local]() { return member + local; };
+ auto lambda4 = [this, &local]() { return member + local; };
+ }
+};
+
+template<typename T>
+void test_template_lambdas() {
+ T value{};
+
+ auto lambda = [=](T x) { return value + x; };
+ // CHECK-MESSAGES: :[[@LINE-1]]:18: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+}
+
+void instantiate_templates() {
+ test_template_lambdas<int>();
+ test_template_lambdas<double>();
+}
>From 689f95b2ba46d6ca319e0735b5f5474bfc14f999 Mon Sep 17 00:00:00 2001
From: jjmarr-amd <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 09:58:59 -0400
Subject: [PATCH 2/7] Fix headers
on-behalf-of: @amd <JJ.Marr at amd.com>
---
.../clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp | 8 ++++++++
.../clang-tidy/bugprone/DefaultLambdaCaptureCheck.h | 8 ++++++++
2 files changed, 16 insertions(+)
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
index 6c95fbc984c5a..0ee65df4b9860 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -1,3 +1,11 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
#include "DefaultLambdaCaptureCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
index ac47c866cfccd..9af861aaf2e93 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.h
@@ -1,3 +1,11 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DEFAULT_LAMBDA_CAPTURE_H
#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DEFAULT_LAMBDA_CAPTURE_H
>From 9991b9ceda13c036e78c0e95978fe16f0bde5959 Mon Sep 17 00:00:00 2001
From: jjmarr-amd <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 10:21:44 -0400
Subject: [PATCH 3/7] remove superfluous comments.
on-behalf-of: @amd <JJ.Marr at amd.com>
---
.../clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp | 2 --
1 file changed, 2 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
index 0ee65df4b9860..6db5cc8fc10bb 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -15,7 +15,6 @@ using namespace clang::ast_matchers;
namespace clang::tidy::bugprone {
void DefaultLambdaCaptureCheck::registerMatchers(MatchFinder *Finder) {
- // Match any lambda expression
Finder->addMatcher(lambdaExpr().bind("lambda"), this);
}
@@ -24,7 +23,6 @@ void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
if (!Lambda)
return;
- // Check if lambda has a default capture
if (Lambda->getCaptureDefault() == LCD_None)
return;
>From db17a4c5a29faa47c69ddb48edb2b0316287c5bc Mon Sep 17 00:00:00 2001
From: "Marr, J J" <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 10:37:30 -0400
Subject: [PATCH 4/7] add new matcher
on-behalf-of: @AMD <JJ.Marr at amd.com>
---
.../bugprone/DefaultLambdaCaptureCheck.cpp | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
index 6db5cc8fc10bb..7fc3e571db220 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -14,8 +14,15 @@ using namespace clang::ast_matchers;
namespace clang::tidy::bugprone {
+namespace {
+AST_MATCHER(LambdaExpr, hasDefaultCapture) {
+ return Node.getCaptureDefault() != LCD_None;
+}
+
+} // namespace
+
void DefaultLambdaCaptureCheck::registerMatchers(MatchFinder *Finder) {
- Finder->addMatcher(lambdaExpr().bind("lambda"), this);
+ Finder->addMatcher(lambdaExpr(hasDefaultCapture()).bind("lambda"), this);
}
void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
@@ -23,8 +30,8 @@ void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
if (!Lambda)
return;
- if (Lambda->getCaptureDefault() == LCD_None)
- return;
+ // No need to check getCaptureDefault() != LCD_None since our custom matcher
+ // hasDefaultCapture() already ensures this condition
SourceLocation DefaultCaptureLoc = Lambda->getCaptureDefaultLoc();
if (DefaultCaptureLoc.isInvalid())
>From 7dcf2162c2a6df4057585e2898bc8feb4536a67c Mon Sep 17 00:00:00 2001
From: "Marr, J J" <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 11:51:27 -0400
Subject: [PATCH 5/7] Replace if with assert
on-behalf-of: @amd <JJ.Marr at amd.com>
---
.../clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
index 7fc3e571db220..4db95101025cc 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -27,11 +27,7 @@ void DefaultLambdaCaptureCheck::registerMatchers(MatchFinder *Finder) {
void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
const auto *Lambda = Result.Nodes.getNodeAs<LambdaExpr>("lambda");
- if (!Lambda)
- return;
-
- // No need to check getCaptureDefault() != LCD_None since our custom matcher
- // hasDefaultCapture() already ensures this condition
+ assert(Lambda);
SourceLocation DefaultCaptureLoc = Lambda->getCaptureDefaultLoc();
if (DefaultCaptureLoc.isInvalid())
>From 176d195cc753b3c95b9a82305e1fe1267e181cfa Mon Sep 17 00:00:00 2001
From: "Marr, J J" <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 11:52:40 -0400
Subject: [PATCH 6/7] Remove specific warning type
on-behalf-of: @amd <JJ.Marr at amd.com>
---
.../bugprone/DefaultLambdaCaptureCheck.cpp | 8 ++-----
.../bugprone/default-lambda-capture.cpp | 24 +++++++++----------
2 files changed, 14 insertions(+), 18 deletions(-)
diff --git a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
index 4db95101025cc..288feb7853e0b 100644
--- a/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/DefaultLambdaCaptureCheck.cpp
@@ -33,12 +33,8 @@ void DefaultLambdaCaptureCheck::check(const MatchFinder::MatchResult &Result) {
if (DefaultCaptureLoc.isInvalid())
return;
- const char *CaptureKind =
- (Lambda->getCaptureDefault() == LCD_ByCopy) ? "by-copy" : "by-reference";
-
- diag(DefaultCaptureLoc, "lambda %0 default capture is discouraged; "
- "prefer to capture specific variables explicitly")
- << CaptureKind;
+ diag(DefaultCaptureLoc, "lambda default captures are discouraged; "
+ "prefer to capture specific variables explicitly");
}
} // namespace clang::tidy::bugprone
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
index f820b4c9b8a0e..010edf11f0a2b 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/default-lambda-capture.cpp
@@ -5,16 +5,16 @@ void test_default_captures() {
int another = 10;
auto lambda1 = [=](int x) { return value + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto lambda2 = [&](int x) { return value + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto lambda3 = [=, &another](int x) { return value + another + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto lambda4 = [&, value](int x) { return value + another + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
}
void test_acceptable_captures() {
@@ -42,10 +42,10 @@ void test_nested_lambdas() {
int inner_var = 3;
auto outer = [=]() {
- // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto inner = [&](int x) { return outer_var + middle_var + inner_var + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
return inner(10);
};
@@ -55,15 +55,15 @@ void test_lambda_returns() {
int a = 1, b = 2, c = 3;
auto create_adder = [=](int x) {
- // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
return [x](int y) { return x + y; }; // Inner lambda is fine - explicit capture
};
auto func1 = [&]() { return a; };
- // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto func2 = [=]() { return b; };
- // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
}
class TestClass {
@@ -74,10 +74,10 @@ class TestClass {
int local = 10;
auto lambda1 = [=]() { return member + local; };
- // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto lambda2 = [&]() { return member + local; };
- // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda by-reference default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
auto lambda3 = [this, local]() { return member + local; };
auto lambda4 = [this, &local]() { return member + local; };
@@ -89,7 +89,7 @@ void test_template_lambdas() {
T value{};
auto lambda = [=](T x) { return value + x; };
- // CHECK-MESSAGES: :[[@LINE-1]]:18: warning: lambda by-copy default capture is discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
+ // CHECK-MESSAGES: :[[@LINE-1]]:18: warning: lambda default captures are discouraged; prefer to capture specific variables explicitly [bugprone-default-lambda-capture]
}
void instantiate_templates() {
>From 5f23dfff1a8368ebca7c2da691201ea1277ad128 Mon Sep 17 00:00:00 2001
From: jjmarr-amd <JJ.Marr at amd.com>
Date: Mon, 22 Sep 2025 12:25:50 -0400
Subject: [PATCH 7/7] Sync documentation to release notes
on-behalf-of: @amd <JJ.Marr at amd.com>
---
.../checks/bugprone/default-lambda-capture.rst | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
index 026acf9f1894b..f1fcf3ec52948 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone/default-lambda-capture.rst
@@ -3,10 +3,11 @@
bugprone-default-lambda-capture
===============================
-Warns when lambda expressions use default capture modes (``[=]`` or ``[&]``)
-instead of explicitly capturing specific variables. Default captures can make
-code less readable and more error-prone by making it unclear which variables
-are being captured and how.
+ Finds lambda expressions that use default capture modes (``[=]`` or ``[&]``)
+ and suggests using explicit captures instead. Default captures can lead to
+ subtle bugs including dangling references with ``[&]``, unnecessary copies
+ with ``[=]``, and make code less maintainable by hiding which variables are
+ actually being captured.
Implements Item 31 of Effective Modern C++ by Scott Meyers.
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