[clang-tidy] Improve `bugprone-exception-escape`'s handling of lambdas #160592

localspook · 2025-09-24T19:48:05Z

Fixes #132605.

llvmbot · 2025-09-24T19:48:41Z

@llvm/pr-subscribers-clang-tidy

Author: Victor Chernyakin (localspook)

Changes

Fixes #132605.

Full diff: https://github.com/llvm/llvm-project/pull/160592.diff

3 Files Affected:

(modified) clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp (+5)
(modified) clang-tools-extra/docs/ReleaseNotes.rst (+5)
(modified) clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp (+38)

diff --git a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
index bdde7249d2796..fd4320eb8144b 100644
--- a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
+++ b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
@@ -595,6 +595,11 @@ ExceptionAnalyzer::throwsException(const Stmt *St,
         Results.merge(DestructorExcs);
       }
     }
+  } else if (const auto *Lambda = dyn_cast<LambdaExpr>(St)) {
+    for (const Stmt *Init : Lambda->capture_inits()) {
+      ExceptionInfo Excs = throwsException(Init, Caught, CallStack);
+      Results.merge(Excs);
+    }
   } else {
     for (const Stmt *Child : St->children()) {
       ExceptionInfo Excs = throwsException(Child, Caught, CallStack);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 7cdff86beeec6..63ea45cb39270 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -210,6 +210,11 @@ Changes in existing checks
   correcting a spelling mistake on its option
   ``NamePrefixSuffixSilenceDissimilarityTreshold``.
 
+- Improved :doc:`bugprone-exception-escape
+  <clang-tidy/checks/bugprone/exception-escape>` check's handling of lambdas:
+  exceptions from captures are now diagnosed, exceptions in the bodies of
+  lambdas that aren't actually invoked are not.
+
 - Improved :doc:`bugprone-infinite-loop
   <clang-tidy/checks/bugprone/infinite-loop>` check by adding detection for
   variables introduced by structured bindings.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
index b10bd1d482867..da200cd6de8df 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
@@ -894,3 +894,41 @@ void pointer_exception_can_not_escape_with_void_handler() noexcept {
   } catch (void *) {
   }
 }
+
+void throw_in_uninvoked_lambda() noexcept {
+  [] { throw 42; };
+}
+
+struct copy_constructor_throws {
+  copy_constructor_throws(const copy_constructor_throws&) { throw 42; }
+};
+
+void throw_in_lambda_default_by_value_capture(const copy_constructor_throws& a) noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda_default_by_value_capture' which should not throw exceptions
+  [=] { a; };
+  // CHECK-MESSAGES: :[[@LINE-6]]:61: note: frame #0: unhandled exception of type 'int' may be thrown in function 'copy_constructor_throws' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:4: note: frame #1: function 'throw_in_lambda_default_by_value_capture' calls function 'copy_constructor_throws' here
+}
+
+void throw_in_lambda_explicit_by_value_capture(const copy_constructor_throws& a) noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda_explicit_by_value_capture' which should not throw exceptions
+  [a] {};
+  // CHECK-MESSAGES: :[[@LINE-13]]:61: note: frame #0: unhandled exception of type 'int' may be thrown in function 'copy_constructor_throws' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:4: note: frame #1: function 'throw_in_lambda_explicit_by_value_capture' calls function 'copy_constructor_throws' here
+}
+
+void no_throw_in_lambda_by_reference_capture(const copy_constructor_throws& a) noexcept {
+  [&] { a; };
+  [&a] {};
+}
+
+void throw_in_uninvoked_lambda() noexcept {
+  [] { throw 42; };
+}
+
+void throw_in_lambda() noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda' which should not throw exceptions
+  [] { throw 42; }();
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: note: frame #0: unhandled exception of type 'int' may be thrown in function 'operator()' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:19: note: frame #1: function 'throw_in_lambda' calls function 'operator()' here
+}

llvmbot · 2025-09-24T19:48:41Z

@llvm/pr-subscribers-clang-tools-extra

Author: Victor Chernyakin (localspook)

Changes

Fixes #132605.

Full diff: https://github.com/llvm/llvm-project/pull/160592.diff

3 Files Affected:

(modified) clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp (+5)
(modified) clang-tools-extra/docs/ReleaseNotes.rst (+5)
(modified) clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp (+38)

diff --git a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
index bdde7249d2796..fd4320eb8144b 100644
--- a/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
+++ b/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp
@@ -595,6 +595,11 @@ ExceptionAnalyzer::throwsException(const Stmt *St,
         Results.merge(DestructorExcs);
       }
     }
+  } else if (const auto *Lambda = dyn_cast<LambdaExpr>(St)) {
+    for (const Stmt *Init : Lambda->capture_inits()) {
+      ExceptionInfo Excs = throwsException(Init, Caught, CallStack);
+      Results.merge(Excs);
+    }
   } else {
     for (const Stmt *Child : St->children()) {
       ExceptionInfo Excs = throwsException(Child, Caught, CallStack);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 7cdff86beeec6..63ea45cb39270 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -210,6 +210,11 @@ Changes in existing checks
   correcting a spelling mistake on its option
   ``NamePrefixSuffixSilenceDissimilarityTreshold``.
 
+- Improved :doc:`bugprone-exception-escape
+  <clang-tidy/checks/bugprone/exception-escape>` check's handling of lambdas:
+  exceptions from captures are now diagnosed, exceptions in the bodies of
+  lambdas that aren't actually invoked are not.
+
 - Improved :doc:`bugprone-infinite-loop
   <clang-tidy/checks/bugprone/infinite-loop>` check by adding detection for
   variables introduced by structured bindings.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
index b10bd1d482867..da200cd6de8df 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/exception-escape.cpp
@@ -894,3 +894,41 @@ void pointer_exception_can_not_escape_with_void_handler() noexcept {
   } catch (void *) {
   }
 }
+
+void throw_in_uninvoked_lambda() noexcept {
+  [] { throw 42; };
+}
+
+struct copy_constructor_throws {
+  copy_constructor_throws(const copy_constructor_throws&) { throw 42; }
+};
+
+void throw_in_lambda_default_by_value_capture(const copy_constructor_throws& a) noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda_default_by_value_capture' which should not throw exceptions
+  [=] { a; };
+  // CHECK-MESSAGES: :[[@LINE-6]]:61: note: frame #0: unhandled exception of type 'int' may be thrown in function 'copy_constructor_throws' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:4: note: frame #1: function 'throw_in_lambda_default_by_value_capture' calls function 'copy_constructor_throws' here
+}
+
+void throw_in_lambda_explicit_by_value_capture(const copy_constructor_throws& a) noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda_explicit_by_value_capture' which should not throw exceptions
+  [a] {};
+  // CHECK-MESSAGES: :[[@LINE-13]]:61: note: frame #0: unhandled exception of type 'int' may be thrown in function 'copy_constructor_throws' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:4: note: frame #1: function 'throw_in_lambda_explicit_by_value_capture' calls function 'copy_constructor_throws' here
+}
+
+void no_throw_in_lambda_by_reference_capture(const copy_constructor_throws& a) noexcept {
+  [&] { a; };
+  [&a] {};
+}
+
+void throw_in_uninvoked_lambda() noexcept {
+  [] { throw 42; };
+}
+
+void throw_in_lambda() noexcept {
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: an exception may be thrown in function 'throw_in_lambda' which should not throw exceptions
+  [] { throw 42; }();
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: note: frame #0: unhandled exception of type 'int' may be thrown in function 'operator()' here
+  // CHECK-MESSAGES: :[[@LINE-2]]:19: note: frame #1: function 'throw_in_lambda' calls function 'operator()' here
+}

vbvictor · 2025-09-24T20:03:51Z

I'd generally wish to add more edge cases for lambdas, just as with functions.
I guess we twist a little how lambdas are processed so we should cover it with more tests

vbvictor · 2025-09-24T20:09:14Z

Also, could you share how we traversed before the fix and now? (If you debugged it)
Given the AST for [] { throw 0; }; (stipped useless part):

    `-LambdaExpr <line:2:5, col:19> '(lambda at line:2:5)'
      |-CXXRecordDecl <col:5> col:5 implicit class definition
      | |-CXXMethodDecl <col:6, col:19> col:5 constexpr operator() 'auto () const -> void' inline
      | | |-CompoundStmt <col:8, col:19>
      | | | `-CXXThrowExpr <col:10, col:16> 'void'
      | | |   `-IntegerLiteral <col:16> 'int' 0
      | | `-InferredNoReturnAttr Implicit
      | |-CXXConversionDecl <col:5, col:19> col:5 implicit constexpr operator void (*)() 'auto (*() const noexcept)() -> void' inline
      | |-CXXMethodDecl <col:5, col:19> col:5 implicit __invoke 'auto () -> void' static inline
      | `-CXXDestructorDecl <col:5> col:5 implicit referenced ~(lambda at line:2:5) 'void () noexcept' inline default trivial
      `-CompoundStmt <col:8, col:19>
        `-CXXThrowExpr <col:10, col:16> 'void'
          `-IntegerLiteral <col:16> 'int' 0

I thought we needed some modelling for calling the lambda in order to properly handle [] {} but in your PR it's different.
Honestly from brief look I don't understand how this fix the problem (guess I'd have to come back a little later for second go)

localspook · 2025-09-24T20:16:45Z

Before this change, the code was going into this branch:

llvm-project/clang-tools-extra/clang-tidy/utils/ExceptionAnalyzer.cpp

Lines 598 to 603 in 738e927

    
           } else { 
        
             for (const Stmt *Child : St->children()) { 
        
               ExceptionInfo Excs = throwsException(Child, Caught, CallStack); 
        
               Results.merge(Excs); 
        
             } 
        
           }

That's the same branch it goes into when it processes a CompoundStmt. In other words, it confused this:

void f() noexcept { [] { throw 42; }; }

for this:

void f() noexcept { { throw 42; } }

My change only affects the lambda expression itself; the actual call to a lambda goes through the existing code paths.

HerrCai0907

LGTM.

could you add some test of

nested lambda
more complex case in lambda e.g. call a function may throw.

localspook · 2025-09-25T15:53:29Z

Added some more tests, tell me if I should add any others

vbvictor

LGTM

llvm#160592) Fixes llvm#132605.

localspook requested review from PiotrZSL, carlosgalvezp, EugeneZelenko, 5chmidti, vbvictor and HerrCai0907 September 24, 2025 19:48

llvmbot added clang-tools-extra clang-tidy labels Sep 24, 2025

localspook force-pushed the exception-escape branch from d9d55b5 to b665234 Compare September 24, 2025 19:55

[clang-tidy] Improve bugprone-exception-escape's handling of lambdas

e0ca1f5

localspook force-pushed the exception-escape branch from b665234 to e0ca1f5 Compare September 24, 2025 20:03

test throw in lambda

5cf9561

localspook force-pushed the exception-escape branch from a6cb241 to 5cf9561 Compare September 24, 2025 20:42

HerrCai0907 approved these changes Sep 25, 2025

View reviewed changes

more tests

7651836

vbvictor approved these changes Sep 25, 2025

View reviewed changes

localspook merged commit 0e93ab8 into llvm:main Sep 26, 2025
11 checks passed

localspook deleted the exception-escape branch September 26, 2025 20:50

mahesh-attarde pushed a commit to mahesh-attarde/llvm-project that referenced this pull request Oct 3, 2025

[clang-tidy] Improve bugprone-exception-escape's handling of lambdas (

dce1f66

llvm#160592) Fixes llvm#132605.

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