From 841215d2015ddd7cc4253129534f435f83cd2883 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109@gmail.com>
Date: Mon, 24 Feb 2025 15:19:29 +0800
Subject: [PATCH 1/2] [Clang] Handle instantiating captures in
 addInstantiatedCapturesToScope()

addInstantiatedCapturesToScope() might be called when transforming a
lambda body. In this situation, it would look into all the lambda's
parents and figure out all the instantiated captures. However, the
instantiated captures are not visible from lambda's class decl until
the lambda is rebuilt (i.e. after the lambda body transform). So this
patch corrects that by also examining the LambdaScopeInfo, serving as
a workaround for not having deferred lambda body instantiation in
Clang 20, to avoid regressing some real-world use cases.
---
 clang/lib/Sema/SemaConcept.cpp              | 25 ++++++++++++++++++++-
 clang/test/SemaTemplate/concepts-lambda.cpp | 18 +++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 8a77cbf8c9477..7fff29f5ca167 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -711,9 +711,32 @@ bool Sema::addInstantiatedCapturesToScope(
 
   unsigned Instantiated = 0;
 
+  // FIXME: This is a workaround for not having deferred lambda body
+  // instantiation.
+  // When transforming a lambda's body, if we encounter another call to an
+  // inline lambda that contains a constraint expression, we add all of the
+  // parent lambda's instantiated captures to the current instantiation scope to
+  // facilitate constraint evaluation. However, these captures don't appear in
+  // the CXXRecordDecl until after the lambda expression is rebuilt, so we
+  // pull them out from the corresponding LSI.
+  LambdaScopeInfo *InstantiatingScope = nullptr;
+  if (LambdaPattern->capture_size() && !LambdaClass->capture_size()) {
+    for (FunctionScopeInfo *Scope : llvm::reverse(FunctionScopes)) {
+      auto *LSI = dyn_cast<LambdaScopeInfo>(Scope);
+      if (!LSI ||
+          LSI->CallOperator->getTemplateInstantiationPattern() != PatternDecl)
+        continue;
+      InstantiatingScope = LSI;
+      break;
+    }
+    assert(InstantiatingScope);
+  }
+
   auto AddSingleCapture = [&](const ValueDecl *CapturedPattern,
                               unsigned Index) {
-    ValueDecl *CapturedVar = LambdaClass->getCapture(Index)->getCapturedVar();
+    ValueDecl *CapturedVar =
+        InstantiatingScope ? InstantiatingScope->Captures[Index].getVariable()
+                           : LambdaClass->getCapture(Index)->getCapturedVar();
     assert(CapturedVar->isInitCapture());
     Scope.InstantiatedLocal(CapturedPattern, CapturedVar);
   };
diff --git a/clang/test/SemaTemplate/concepts-lambda.cpp b/clang/test/SemaTemplate/concepts-lambda.cpp
index 306f86cfcb28f..dcb09c76d26b6 100644
--- a/clang/test/SemaTemplate/concepts-lambda.cpp
+++ b/clang/test/SemaTemplate/concepts-lambda.cpp
@@ -307,3 +307,21 @@ void test() {
 }
 
 }
+
+namespace GH128175 {
+
+template <class> void f() {
+  [i{0}] {
+    [&] {
+      [&] {
+        []()
+          requires true
+        {}();
+      }();
+    }();
+  }();
+}
+
+template void f<int>();
+
+}

From 8d442a77e25fb3534548bf8d69ba35a9511cf979 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109@gmail.com>
Date: Tue, 25 Feb 2025 11:35:24 +0800
Subject: [PATCH 2/2] Improve comments

---
 clang/lib/Sema/SemaConcept.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 7fff29f5ca167..a7b609f7f3ce4 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -713,9 +713,9 @@ bool Sema::addInstantiatedCapturesToScope(
 
   // FIXME: This is a workaround for not having deferred lambda body
   // instantiation.
-  // When transforming a lambda's body, if we encounter another call to an
-  // inline lambda that contains a constraint expression, we add all of the
-  // parent lambda's instantiated captures to the current instantiation scope to
+  // When transforming a lambda's body, if we encounter another call to a
+  // nested lambda that contains a constraint expression, we add all of the
+  // outer lambda's instantiated captures to the current instantiation scope to
   // facilitate constraint evaluation. However, these captures don't appear in
   // the CXXRecordDecl until after the lambda expression is rebuilt, so we
   // pull them out from the corresponding LSI.