[clang] [Clang] prevent assertion failure by avoiding always-dependent lambdas in constraint-related nested scopes (PR #173776)

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk updated https://github.com/llvm/llvm-project/pull/173776

>From 0e6bc4ff732cfcaf5d2ba445dd25f8064da128e3 Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <oleksandr.tarasiuk at outlook.com>
Date: Sun, 28 Dec 2025 14:15:42 +0200
Subject: [PATCH 1/3] [Clang] prevent assertion failure by avoiding
 always-dependent lambdas in constraint-related nested scopes

---
 clang/docs/ReleaseNotes.rst               |  1 +
 clang/lib/Sema/SemaLambda.cpp             | 14 +++++++++-----
 clang/test/SemaCXX/lambda-unevaluated.cpp |  9 ++++++++-
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2319ff13f7864..1ae44bf610221 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -623,6 +623,7 @@ Bug Fixes to C++ Support
 - Fix the result of ``__is_pointer_interconvertible_base_of`` when arguments are qualified and passed via template parameters. (#GH135273)
 - Fixed a crash when evaluating nested requirements in requires-expressions that reference invented parameters. (#GH166325)
 - Fixed a crash when standard comparison categories (e.g. ``std::partial_ordering``) are defined with incorrect static member types. (#GH170015) (#GH56571)
+- Fixed an assertion failure triggered by nested lambdas during capture handling. (#GH172814)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 81ec0b18dedfd..53f2612a46bf4 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1102,7 +1102,7 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
       CXXRecordDecl::LDK_Unknown;
   if (CurScope->getTemplateParamParent() != nullptr) {
     LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
-  } else if (Scope *P = CurScope->getParent()) {
+  } else if (Scope *ParentScope = CurScope->getParent()) {
     // Given a lambda defined inside a requires expression,
     //
     // struct S {
@@ -1113,10 +1113,14 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
     // The parameter var is not injected into the function Decl at the point of
     // parsing lambda. In such scenarios, perceiving it as dependent could
     // result in the constraint being evaluated, which matches what GCC does.
-    while (P->getEntity() && P->getEntity()->isRequiresExprBody())
-      P = P->getParent();
-    if (P->isFunctionDeclarationScope() &&
-        llvm::any_of(P->decls(), [](Decl *D) {
+    Scope *LookupScope = ParentScope;
+    while (LookupScope->getEntity() &&
+           LookupScope->getEntity()->isRequiresExprBody())
+      LookupScope = LookupScope->getParent();
+
+    if (LookupScope != ParentScope &&
+        LookupScope->isFunctionDeclarationScope() &&
+        llvm::any_of(LookupScope->decls(), [](Decl *D) {
           return isa<ParmVarDecl>(D) &&
                  cast<ParmVarDecl>(D)->getType()->isTemplateTypeParmType();
         }))
diff --git a/clang/test/SemaCXX/lambda-unevaluated.cpp b/clang/test/SemaCXX/lambda-unevaluated.cpp
index 9289fc9ec1243..e3dffa00cd39a 100644
--- a/clang/test/SemaCXX/lambda-unevaluated.cpp
+++ b/clang/test/SemaCXX/lambda-unevaluated.cpp
@@ -1,7 +1,6 @@
 // RUN: %clang_cc1 -std=c++20 %s -Wno-c++23-extensions -verify
 // RUN: %clang_cc1 -std=c++23 %s -verify
 
-
 template <auto> struct Nothing {};
 Nothing<[]() { return 0; }()> nothing;
 
@@ -282,3 +281,11 @@ static_assert(__is_same_as(int, helper<int>));
 
 
 } // namespace GH138018
+
+namespace GH172814 {
+auto t() {
+  int x = 0;
+  return [](auto w=[&] { x += w(); }); // expected-error {{lambda expression in default argument cannot capture any entity}} \
+                                       // expected-error {{expected body of lambda expression}}
+};
+}

>From d50bbaa5f931829961d6b5c24c4bfbe22645b541 Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <oleksandr.tarasiuk at outlook.com>
Date: Mon, 29 Dec 2025 11:23:51 +0200
Subject: [PATCH 2/3] add additional test case

---
 clang/test/SemaCXX/lambda-unevaluated.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/clang/test/SemaCXX/lambda-unevaluated.cpp b/clang/test/SemaCXX/lambda-unevaluated.cpp
index e3dffa00cd39a..d02eabf11edb4 100644
--- a/clang/test/SemaCXX/lambda-unevaluated.cpp
+++ b/clang/test/SemaCXX/lambda-unevaluated.cpp
@@ -283,9 +283,13 @@ static_assert(__is_same_as(int, helper<int>));
 } // namespace GH138018
 
 namespace GH172814 {
-auto t() {
+auto t1() {
   int x = 0;
   return [](auto w=[&] { x += w(); }); // expected-error {{lambda expression in default argument cannot capture any entity}} \
                                        // expected-error {{expected body of lambda expression}}
 };
+auto t2() {
+  int x = 0;
+  return [](auto w = [&] { return x; }) { }; // expected-error {{lambda expression in default argument cannot capture any entity}}
+};
 }

>From 1121701a83adfe40f9958bbd13e840b5e537fe82 Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <oleksandr.tarasiuk at outlook.com>
Date: Mon, 29 Dec 2025 14:03:46 +0200
Subject: [PATCH 3/3] remove unnecessary test

---
 clang/test/SemaCXX/lambda-unevaluated.cpp | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/clang/test/SemaCXX/lambda-unevaluated.cpp b/clang/test/SemaCXX/lambda-unevaluated.cpp
index d02eabf11edb4..55a4bc7aff1b5 100644
--- a/clang/test/SemaCXX/lambda-unevaluated.cpp
+++ b/clang/test/SemaCXX/lambda-unevaluated.cpp
@@ -283,12 +283,7 @@ static_assert(__is_same_as(int, helper<int>));
 } // namespace GH138018
 
 namespace GH172814 {
-auto t1() {
-  int x = 0;
-  return [](auto w=[&] { x += w(); }); // expected-error {{lambda expression in default argument cannot capture any entity}} \
-                                       // expected-error {{expected body of lambda expression}}
-};
-auto t2() {
+auto t() {
   int x = 0;
   return [](auto w = [&] { return x; }) { }; // expected-error {{lambda expression in default argument cannot capture any entity}}
 };