[Clang] Delegate part of SetupConstraintScope's job to LambdaScopeForCallOperatorInstantiationRAII (#123687)

Now that the RAII object has a dedicate logic for handling nested
lambdas, where the inner lambda could reference any
captures/variables/parameters from the outer lambda, we can shift the
responsibility for managing lambdas away from SetupConstraintScope().

I think this also makes the structure clearer.

Fixes #123441

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -965,6 +965,7 @@ Bug Fixes to C++ Support
 - Fixed a crash caused by the incorrect construction of template arguments for CTAD alias guides when type
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
+- Fixed a nested lambda substitution issue for constraint evaluation. (#GH123441)
 
 
 Bug Fixes to AST Handling

clang/include/clang/Sema/Sema.h

@@ -13841,6 +13841,13 @@ class Sema final : public SemaBase {
       LocalInstantiationScope &Scope,
       const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  /// Introduce the instantiated captures of the lambda into the local
+  /// instantiation scope.
+  bool addInstantiatedCapturesToScope(
+      FunctionDecl *Function, const FunctionDecl *PatternDecl,
+      LocalInstantiationScope &Scope,
+      const MultiLevelTemplateArgumentList &TemplateArgs);
+
   int ParsingClassDepth = 0;
 
   class SavePendingParsedClassStateRAII {
@@ -14521,16 +14528,9 @@ class Sema final : public SemaBase {
   // The current stack of constraint satisfactions, so we can exit-early.
   llvm::SmallVector<SatisfactionStackEntryTy, 10> SatisfactionStack;
 
-  /// Introduce the instantiated captures of the lambda into the local
-  /// instantiation scope.
-  bool addInstantiatedCapturesToScope(
-      FunctionDecl *Function, const FunctionDecl *PatternDecl,
-      LocalInstantiationScope &Scope,
-      const MultiLevelTemplateArgumentList &TemplateArgs);
-
-  /// Used by SetupConstraintCheckingTemplateArgumentsAndScope to recursively(in
-  /// the case of lambdas) set up the LocalInstantiationScope of the current
-  /// function.
+  /// Used by SetupConstraintCheckingTemplateArgumentsAndScope to set up the
+  /// LocalInstantiationScope of the current non-lambda function. For lambdas,
+  /// use LambdaScopeForCallOperatorInstantiationRAII.
   bool
   SetupConstraintScope(FunctionDecl *FD,
                        std::optional<ArrayRef<TemplateArgument>> TemplateArgs,

clang/lib/Sema/SemaConcept.cpp

@@ -752,6 +752,9 @@ bool Sema::SetupConstraintScope(
     FunctionDecl *FD, std::optional<ArrayRef<TemplateArgument>> TemplateArgs,
     const MultiLevelTemplateArgumentList &MLTAL,
     LocalInstantiationScope &Scope) {
+  assert(!isLambdaCallOperator(FD) &&
+         "Use LambdaScopeForCallOperatorInstantiationRAII to handle lambda "
+         "instantiations");
   if (FD->isTemplateInstantiation() && FD->getPrimaryTemplate()) {
     FunctionTemplateDecl *PrimaryTemplate = FD->getPrimaryTemplate();
     InstantiatingTemplate Inst(
@@ -777,14 +780,8 @@ bool Sema::SetupConstraintScope(
 
     // If this is a member function, make sure we get the parameters that
     // reference the original primary template.
-    // We walk up the instantiated template chain so that nested lambdas get
-    // handled properly.
-    // We should only collect instantiated parameters from the primary template.
-    // Otherwise, we may have mismatched template parameter depth!
     if (FunctionTemplateDecl *FromMemTempl =
             PrimaryTemplate->getInstantiatedFromMemberTemplate()) {
-      while (FromMemTempl->getInstantiatedFromMemberTemplate())
-        FromMemTempl = FromMemTempl->getInstantiatedFromMemberTemplate();
       if (addInstantiatedParametersToScope(FD, FromMemTempl->getTemplatedDecl(),
                                            Scope, MLTAL))
         return true;
@@ -834,6 +831,9 @@ Sema::SetupConstraintCheckingTemplateArgumentsAndScope(
                                    /*RelativeToPrimary=*/true,
                                    /*Pattern=*/nullptr,
                                    /*ForConstraintInstantiation=*/true);
+  // Lambdas are handled by LambdaScopeForCallOperatorInstantiationRAII.
+  if (isLambdaCallOperator(FD))
+    return MLTAL;
   if (SetupConstraintScope(FD, TemplateArgs, MLTAL, Scope))
     return std::nullopt;
 

clang/lib/Sema/SemaLambda.cpp

@@ -2408,35 +2408,31 @@ Sema::LambdaScopeForCallOperatorInstantiationRAII::
   if (!ShouldAddDeclsFromParentScope)
     return;
 
-  FunctionDecl *InnermostFD = FD, *InnermostFDPattern = FDPattern;
   llvm::SmallVector<std::pair<FunctionDecl *, FunctionDecl *>, 4>
-      ParentInstantiations;
-  while (true) {
+      InstantiationAndPatterns;
+  while (FDPattern && FD) {
+    InstantiationAndPatterns.emplace_back(FDPattern, FD);
+
     FDPattern =
         dyn_cast<FunctionDecl>(getLambdaAwareParentOfDeclContext(FDPattern));
     FD = dyn_cast<FunctionDecl>(getLambdaAwareParentOfDeclContext(FD));
-
-    if (!FDPattern || !FD)
-      break;
-
-    ParentInstantiations.emplace_back(FDPattern, FD);
   }
 
   // Add instantiated parameters and local vars to scopes, starting from the
   // outermost lambda to the innermost lambda. This ordering ensures that
-  // parameters in inner lambdas can correctly depend on those defined
-  // in outer lambdas, e.g. auto L = [](auto... x) {
-  //   return [](decltype(x)... y) { }; // `y` depends on `x`
-  // };
+  // the outer instantiations can be found when referenced from within inner
+  // lambdas.
+  //
+  //   auto L = [](auto... x) {
+  //     return [](decltype(x)... y) { }; // Instantiating y needs x
+  //   };
+  //
 
-  for (const auto &[FDPattern, FD] : llvm::reverse(ParentInstantiations)) {
+  for (auto [FDPattern, FD] : llvm::reverse(InstantiationAndPatterns)) {
     SemaRef.addInstantiatedParametersToScope(FD, FDPattern, Scope, MLTAL);
     SemaRef.addInstantiatedLocalVarsToScope(FD, FDPattern, Scope);
 
     if (isLambdaCallOperator(FD))
       SemaRef.addInstantiatedCapturesToScope(FD, FDPattern, Scope, MLTAL);
   }
-
-  SemaRef.addInstantiatedCapturesToScope(InnermostFD, InnermostFDPattern, Scope,
-                                         MLTAL);
 }

clang/test/SemaTemplate/concepts-lambda.cpp

@@ -294,3 +294,16 @@ void foo() {
 }
 
 } // namespace GH110721
+
+namespace GH123441 {
+
+void test() {
+  auto L = [](auto... x) {
+    return [](decltype(x)... y)
+      requires true
+    {};
+  };
+  L(0, 1)(1, 2);
+}
+
+}