[Clang] Fix a lambda pattern comparison mismatch after ecc7e6ce4 (#133863)

In ecc7e6ce4, we tried to inspect the `LambdaScopeInfo` on stack to
recover the instantiating lambda captures. However, there was a mismatch
in how we compared the pattern declarations of lambdas: the constraint
instantiation used a tailored `getPatternFunctionDecl()` which is
localized in SemaLambda that finds the very primal template declaration
of a lambda, while `FunctionDecl::getTemplateInstantiationPattern` finds
the latest template pattern of a lambda. This difference causes issues
when lambdas are nested, as we always want the primary template
declaration.

This corrects that by moving `Sema::addInstantiatedCapturesToScope` from
SemaConcept to SemaLambda, allowing it to use the localized version of
`getPatternFunctionDecl`.

It is also worth exploring to coalesce the implementation of
`getPatternFunctionDecl` with
`FunctionDecl::getTemplateInstantiationPattern`. But I’m leaving that
for the future, as I’d like to backport this fix (ecc7e6ce4 made the
issue more visible in clang 20, sorry!), and changing Sema’s ABI would
not be suitable in that regards. Hence, no release note.

Fixes https://github.com/llvm/llvm-project/issues/133719

3 files changed, 83 insertions, 69 deletions

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 5e1cd62..16f9e3d 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -697,75 +697,6 @@ bool Sema::CheckConstraintSatisfaction(
       .isInvalid();
 }
 
-bool Sema::addInstantiatedCapturesToScope(
-    FunctionDecl *Function, const FunctionDecl *PatternDecl,
-    LocalInstantiationScope &Scope,
-    const MultiLevelTemplateArgumentList &TemplateArgs) {
-  const auto *LambdaClass = cast<CXXMethodDecl>(Function)->getParent();
-  const auto *LambdaPattern = cast<CXXMethodDecl>(PatternDecl)->getParent();
-
-  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 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.
-  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 =
-        InstantiatingScope ? InstantiatingScope->Captures[Index].getVariable()
-                           : LambdaClass->getCapture(Index)->getCapturedVar();
-    assert(CapturedVar->isInitCapture());
-    Scope.InstantiatedLocal(CapturedPattern, CapturedVar);
-  };
-
-  for (const LambdaCapture &CapturePattern : LambdaPattern->captures()) {
-    if (!CapturePattern.capturesVariable()) {
-      Instantiated++;
-      continue;
-    }
-    ValueDecl *CapturedPattern = CapturePattern.getCapturedVar();
-
-    if (!CapturedPattern->isInitCapture()) {
-      Instantiated++;
-      continue;
-    }
-
-    if (!CapturedPattern->isParameterPack()) {
-      AddSingleCapture(CapturedPattern, Instantiated++);
-    } else {
-      Scope.MakeInstantiatedLocalArgPack(CapturedPattern);
-      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
-      SemaRef.collectUnexpandedParameterPacks(
-          dyn_cast<VarDecl>(CapturedPattern)->getInit(), Unexpanded);
-      auto NumArgumentsInExpansion =
-          getNumArgumentsInExpansionFromUnexpanded(Unexpanded, TemplateArgs);
-      if (!NumArgumentsInExpansion)
-        continue;
-      for (unsigned Arg = 0; Arg < *NumArgumentsInExpansion; ++Arg)
-        AddSingleCapture(CapturedPattern, Instantiated++);
-    }
-  }
-  return false;
-}
-
 bool Sema::SetupConstraintScope(
     FunctionDecl *FD, std::optional<ArrayRef<TemplateArgument>> TemplateArgs,
     const MultiLevelTemplateArgumentList &MLTAL,
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 292406f..6f114b7 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -2412,6 +2412,74 @@ static FunctionDecl *getPatternFunctionDecl(FunctionDecl *FD) {
   return FTD->getTemplatedDecl();
 }
 
+bool Sema::addInstantiatedCapturesToScope(
+    FunctionDecl *Function, const FunctionDecl *PatternDecl,
+    LocalInstantiationScope &Scope,
+    const MultiLevelTemplateArgumentList &TemplateArgs) {
+  const auto *LambdaClass = cast<CXXMethodDecl>(Function)->getParent();
+  const auto *LambdaPattern = cast<CXXMethodDecl>(PatternDecl)->getParent();
+
+  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 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.
+  LambdaScopeInfo *InstantiatingScope = nullptr;
+  if (LambdaPattern->capture_size() && !LambdaClass->capture_size()) {
+    for (FunctionScopeInfo *Scope : llvm::reverse(FunctionScopes)) {
+      auto *LSI = dyn_cast<LambdaScopeInfo>(Scope);
+      if (!LSI || getPatternFunctionDecl(LSI->CallOperator) != PatternDecl)
+        continue;
+      InstantiatingScope = LSI;
+      break;
+    }
+    assert(InstantiatingScope);
+  }
+
+  auto AddSingleCapture = [&](const ValueDecl *CapturedPattern,
+                              unsigned Index) {
+    ValueDecl *CapturedVar =
+        InstantiatingScope ? InstantiatingScope->Captures[Index].getVariable()
+                           : LambdaClass->getCapture(Index)->getCapturedVar();
+    assert(CapturedVar->isInitCapture());
+    Scope.InstantiatedLocal(CapturedPattern, CapturedVar);
+  };
+
+  for (const LambdaCapture &CapturePattern : LambdaPattern->captures()) {
+    if (!CapturePattern.capturesVariable()) {
+      Instantiated++;
+      continue;
+    }
+    ValueDecl *CapturedPattern = CapturePattern.getCapturedVar();
+
+    if (!CapturedPattern->isInitCapture()) {
+      Instantiated++;
+      continue;
+    }
+
+    if (!CapturedPattern->isParameterPack()) {
+      AddSingleCapture(CapturedPattern, Instantiated++);
+    } else {
+      Scope.MakeInstantiatedLocalArgPack(CapturedPattern);
+      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+      SemaRef.collectUnexpandedParameterPacks(
+          dyn_cast<VarDecl>(CapturedPattern)->getInit(), Unexpanded);
+      auto NumArgumentsInExpansion =
+          getNumArgumentsInExpansionFromUnexpanded(Unexpanded, TemplateArgs);
+      if (!NumArgumentsInExpansion)
+        continue;
+      for (unsigned Arg = 0; Arg < *NumArgumentsInExpansion; ++Arg)
+        AddSingleCapture(CapturedPattern, Instantiated++);
+    }
+  }
+  return false;
+}
+
 Sema::LambdaScopeForCallOperatorInstantiationRAII::
     LambdaScopeForCallOperatorInstantiationRAII(
         Sema &SemaRef, FunctionDecl *FD, MultiLevelTemplateArgumentList MLTAL,
diff --git a/clang/test/SemaTemplate/concepts-lambda.cpp b/clang/test/SemaTemplate/concepts-lambda.cpp
index dcb09c7..1f67c25 100644
--- a/clang/test/SemaTemplate/concepts-lambda.cpp
+++ b/clang/test/SemaTemplate/concepts-lambda.cpp
@@ -325,3 +325,18 @@ template <class> void f() {
 template void f<int>();
 
 }
+
+namespace GH133719 {
+
+template <class T>
+constexpr auto f{[] (auto arg) {
+  return [a{arg}] {
+      [] () requires true {}();
+  };
+}};
+
+void foo() {
+  f<int>(0);
+}
+
+}