[Clang][Sema] Fix the lambda call expression inside of a type alias declaration

clang/docs/ReleaseNotes.rst

@@ -360,6 +360,8 @@ Bug Fixes to C++ Support
   when one of the function had more specialized templates.
   Fixes (`#82509 <https://github.com/llvm/llvm-project/issues/82509>`_)
   and (`#74494 <https://github.com/llvm/llvm-project/issues/74494>`_)
+- Clang now supports direct lambda calls inside of a type alias template declarations.
+  This addresses (#GH70601), (#GH76674), (#GH79555), (#GH81145) and (#GH82104).
 - Allow access to a public template alias declaration that refers to friend's
   private nested type. (#GH25708).
 

clang/include/clang/AST/DeclCXX.h

@@ -1869,6 +1869,10 @@ class CXXRecordDecl : public RecordDecl {
     DL.MethodTyInfo = TS;
   }
 
+  void setLambdaDependencyKind(unsigned Kind) {
+    getLambdaData().DependencyKind = Kind;
+  }
+
   void setLambdaIsGeneric(bool IsGeneric) {
     assert(DefinitionData && DefinitionData->IsLambda &&
            "setting lambda property of non-lambda class");

clang/include/clang/Sema/Sema.h

@@ -10153,6 +10153,9 @@ class Sema final {
 
       /// We are building deduction guides for a class.
       BuildingDeductionGuides,
+
+      /// We are instantiating a type alias template declaration.
+      TypeAliasTemplateInstantiation,
     } Kind;
 
     /// Was the enclosing context a non-instantiation SFINAE context?
@@ -10242,6 +10245,12 @@ class Sema final {
                           FunctionDecl *Entity, ExceptionSpecification,
                           SourceRange InstantiationRange = SourceRange());
 
+    /// Note that we are instantiating a type alias template declaration.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          TypeAliasTemplateDecl *Template,
+                          ArrayRef<TemplateArgument> TemplateArgs,
+                          SourceRange InstantiationRange = SourceRange());
+
     /// Note that we are instantiating a default argument in a
     /// template-id.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,

clang/lib/Frontend/FrontendActions.cpp

@@ -450,6 +450,8 @@ class DefaultTemplateInstCallback : public TemplateInstantiationCallback {
       return "BuildingBuiltinDumpStructCall";
     case CodeSynthesisContext::BuildingDeductionGuides:
       return "BuildingDeductionGuides";
+    case Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation:
+      return "TypeAliasTemplateInstantiation";
     }
     return "";
   }

clang/lib/Sema/SemaConcept.cpp

@@ -615,10 +615,12 @@ bool Sema::SetupConstraintScope(
     // reference the original primary template.
     // We walk up the instantiated template chain so that nested lambdas get
     // handled properly.
-    for (FunctionTemplateDecl *FromMemTempl =
-             PrimaryTemplate->getInstantiatedFromMemberTemplate();
-         FromMemTempl;
-         FromMemTempl = FromMemTempl->getInstantiatedFromMemberTemplate()) {
+    // 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;

clang/lib/Sema/SemaTemplate.cpp

@@ -4343,6 +4343,11 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Inst.isInvalid())
       return QualType();
 
+    InstantiatingTemplate InstTemplate(
+        *this, /*PointOfInstantiation=*/AliasTemplate->getBeginLoc(),
+        /*Template=*/AliasTemplate,
+        /*TemplateArgs=*/TemplateArgLists.getInnermost());
+
     std::optional<ContextRAII> SavedContext;
     if (!AliasTemplate->getDeclContext()->isFileContext())
       SavedContext.emplace(*this, AliasTemplate->getDeclContext());

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -80,6 +80,81 @@ struct Response {
     return R;
   }
 };
+
+// Retrieve the primary template for a lambda call operator. It's
+// unfortunate that we only have the mappings of call operators rather
+// than lambda classes.
+const FunctionDecl *
+getPrimaryTemplateOfGenericLambda(const FunctionDecl *LambdaCallOperator) {
+  while (true) {
+    if (auto *FTD = dyn_cast_if_present<FunctionTemplateDecl>(
+            LambdaCallOperator->getDescribedTemplate());
+        FTD && FTD->getInstantiatedFromMemberTemplate()) {
+      LambdaCallOperator =
+          FTD->getInstantiatedFromMemberTemplate()->getTemplatedDecl();
+    } else if (auto *Prev = cast<CXXMethodDecl>(LambdaCallOperator)
+                                ->getInstantiatedFromMemberFunction())
+      LambdaCallOperator = Prev;
+    else
+      break;
+  }
+  return LambdaCallOperator;
+}
+
+struct EnclosingTypeAliasTemplateDetails {
+  TypeAliasTemplateDecl *Template = nullptr;
+  TypeAliasTemplateDecl *PrimaryTypeAliasDecl = nullptr;
+  ArrayRef<TemplateArgument> AssociatedTemplateArguments;
+
+  explicit operator bool() noexcept { return Template; }
+};
+
+// Find the enclosing type alias template Decl from CodeSynthesisContexts, as
+// well as its primary template and instantiating template arguments.
+EnclosingTypeAliasTemplateDetails
+getEnclosingTypeAliasTemplateDecl(Sema &SemaRef) {
+  for (auto &CSC : llvm::reverse(SemaRef.CodeSynthesisContexts)) {
+    if (CSC.Kind != Sema::CodeSynthesisContext::SynthesisKind::
+                        TypeAliasTemplateInstantiation)
+      continue;
+    EnclosingTypeAliasTemplateDetails Result;
+    auto *TATD = cast<TypeAliasTemplateDecl>(CSC.Entity),
+         *Next = TATD->getInstantiatedFromMemberTemplate();
+    Result = {
+        /*Template=*/TATD,
+        /*PrimaryTypeAliasDecl=*/TATD,
+        /*AssociatedTemplateArguments=*/CSC.template_arguments(),
+    };
+    while (Next) {
+      Result.PrimaryTypeAliasDecl = Next;
+      Next = Next->getInstantiatedFromMemberTemplate();
+    }
+    return Result;
+  }
+  return {};
+}
+
+// Check if we are currently inside of a lambda expression that is
+// surrounded by a using alias declaration. e.g.
+//   template <class> using type = decltype([](auto) { ^ }());
+// By checking if:
+//  1. The lambda expression and the using alias declaration share the
+//  same declaration context.
+//  2. They have the same template depth.
+// We have to do so since a TypeAliasTemplateDecl (or a TypeAliasDecl) is never
+// a DeclContext, nor does it have an associated specialization Decl from which
+// we could collect these template arguments.
+bool isLambdaEnclosedByTypeAliasDecl(
+    const FunctionDecl *PrimaryLambdaCallOperator,
+    const TypeAliasTemplateDecl *PrimaryTypeAliasDecl) {
+  return cast<CXXRecordDecl>(PrimaryLambdaCallOperator->getDeclContext())
+                 ->getTemplateDepth() ==
+             PrimaryTypeAliasDecl->getTemplateDepth() &&
+         getLambdaAwareParentOfDeclContext(
+             const_cast<FunctionDecl *>(PrimaryLambdaCallOperator)) ==
+             PrimaryTypeAliasDecl->getDeclContext();
+}
+
 // Add template arguments from a variable template instantiation.
 Response
 HandleVarTemplateSpec(const VarTemplateSpecializationDecl *VarTemplSpec,
@@ -176,7 +251,7 @@ HandleClassTemplateSpec(const ClassTemplateSpecializationDecl *ClassTemplSpec,
   return Response::UseNextDecl(ClassTemplSpec);
 }
 
-Response HandleFunction(const FunctionDecl *Function,
+Response HandleFunction(Sema &SemaRef, const FunctionDecl *Function,
                         MultiLevelTemplateArgumentList &Result,
                         const FunctionDecl *Pattern, bool RelativeToPrimary,
                         bool ForConstraintInstantiation) {
@@ -207,8 +282,23 @@ Response HandleFunction(const FunctionDecl *Function,
 
     // If this function is a generic lambda specialization, we are done.
     if (!ForConstraintInstantiation &&
-        isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
+        isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function)) {
+      // TypeAliasTemplateDecls should be taken into account, e.g.
+      // when we're deducing the return type of a lambda.
+      //
+      // template <class> int Value = 0;
+      // template <class T>
+      // using T = decltype([]<int U = 0>() { return Value<T>; }());
+      //
+      if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(SemaRef)) {
+        if (isLambdaEnclosedByTypeAliasDecl(
+                /*PrimaryLambdaCallOperator=*/getPrimaryTemplateOfGenericLambda(
+                    Function),
+                /*PrimaryTypeAliasDecl=*/TypeAlias.PrimaryTypeAliasDecl))
+          return Response::UseNextDecl(Function);
+      }
       return Response::Done();
+    }
 
   } else if (Function->getDescribedFunctionTemplate()) {
     assert(
@@ -283,7 +373,7 @@ Response HandleFunctionTemplateDecl(const FunctionTemplateDecl *FTD,
   return Response::ChangeDecl(FTD->getLexicalDeclContext());
 }
 
-Response HandleRecordDecl(const CXXRecordDecl *Rec,
+Response HandleRecordDecl(Sema &SemaRef, const CXXRecordDecl *Rec,
                           MultiLevelTemplateArgumentList &Result,
                           ASTContext &Context,
                           bool ForConstraintInstantiation) {
@@ -312,11 +402,39 @@ Response HandleRecordDecl(const CXXRecordDecl *Rec,
     return Response::ChangeDecl(Rec->getLexicalDeclContext());
   }
 
-  // This is to make sure we pick up the VarTemplateSpecializationDecl that this
-  // lambda is defined inside of.
-  if (Rec->isLambda())
+  // This is to make sure we pick up the VarTemplateSpecializationDecl or the
+  // TypeAliasTemplateDecl that this lambda is defined inside of.
+  if (Rec->isLambda()) {
     if (const Decl *LCD = Rec->getLambdaContextDecl())
       return Response::ChangeDecl(LCD);
+    // Retrieve the template arguments for a using alias declaration.
+    // This is necessary for constraint checking, since we always keep
+    // constraints relative to the primary template.
+    if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(SemaRef)) {
+      const FunctionDecl *PrimaryLambdaCallOperator =
+          getPrimaryTemplateOfGenericLambda(Rec->getLambdaCallOperator());
+      if (isLambdaEnclosedByTypeAliasDecl(PrimaryLambdaCallOperator,
+                                          TypeAlias.PrimaryTypeAliasDecl)) {
+        Result.addOuterTemplateArguments(TypeAlias.Template,
+                                         TypeAlias.AssociatedTemplateArguments,
+                                         /*Final=*/false);
+        // Visit the parent of the current type alias declaration rather than
+        // the lambda thereof.
+        // E.g., in the following example:
+        // struct S {
+        //  template <class> using T = decltype([]<Concept> {} ());
+        // };
+        // void foo() {
+        //   S::T var;
+        // }
+        // The instantiated lambda expression (which we're visiting at 'var')
+        // has a function DeclContext 'foo' rather than the Record DeclContext
+        // S. This seems to be an oversight to me that we may want to set a
+        // Sema Context from the CXXScopeSpec before substituting into T.
+        return Response::ChangeDecl(TypeAlias.Template->getDeclContext());
+      }
+    }
+  }
 
   return Response::UseNextDecl(Rec);
 }
@@ -410,10 +528,11 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
       R = HandleClassTemplateSpec(ClassTemplSpec, Result,
                                   SkipForSpecialization);
     } else if (const auto *Function = dyn_cast<FunctionDecl>(CurDecl)) {
-      R = HandleFunction(Function, Result, Pattern, RelativeToPrimary,
+      R = HandleFunction(*this, Function, Result, Pattern, RelativeToPrimary,
                          ForConstraintInstantiation);
     } else if (const auto *Rec = dyn_cast<CXXRecordDecl>(CurDecl)) {
-      R = HandleRecordDecl(Rec, Result, Context, ForConstraintInstantiation);
+      R = HandleRecordDecl(*this, Rec, Result, Context,
+                           ForConstraintInstantiation);
     } else if (const auto *CSD =
                    dyn_cast<ImplicitConceptSpecializationDecl>(CurDecl)) {
       R = HandleImplicitConceptSpecializationDecl(CSD, Result);
@@ -470,6 +589,7 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case BuildingBuiltinDumpStructCall:
   case LambdaExpressionSubstitution:
   case BuildingDeductionGuides:
+  case TypeAliasTemplateInstantiation:
     return false;
 
   // This function should never be called when Kind's value is Memoization.
@@ -615,6 +735,15 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
           PointOfInstantiation, InstantiationRange, Param, Template,
           TemplateArgs) {}
 
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    TypeAliasTemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/Template,
+          /*Template=*/nullptr, TemplateArgs) {}
+
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation, TemplateDecl *Template,
     NamedDecl *Param, ArrayRef<TemplateArgument> TemplateArgs,
@@ -1132,6 +1261,8 @@ void Sema::PrintInstantiationStack() {
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_building_deduction_guide_here);
       break;
+    case CodeSynthesisContext::TypeAliasTemplateInstantiation:
+      break;
     }
   }
 }
@@ -1209,6 +1340,7 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
       break;
 
     case CodeSynthesisContext::Memoization:
+    case CodeSynthesisContext::TypeAliasTemplateInstantiation:
       break;
     }
 
@@ -1534,6 +1666,18 @@ namespace {
                                            SubstTemplateTypeParmPackTypeLoc TL,
                                            bool SuppressObjCLifetime);
 
+    CXXRecordDecl::LambdaDependencyKind
+    ComputeLambdaDependency(LambdaScopeInfo *LSI) {
+      auto &CCS = SemaRef.CodeSynthesisContexts.back();
+      if (CCS.Kind ==
+          Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation) {
+        unsigned TypeAliasDeclDepth = CCS.Entity->getTemplateDepth();
+        if (TypeAliasDeclDepth >= TemplateArgs.getNumSubstitutedLevels())
+          return CXXRecordDecl::LambdaDependencyKind::LDK_AlwaysDependent;
+      }
+      return inherited::ComputeLambdaDependency(LSI);
+    }
+
     ExprResult TransformLambdaExpr(LambdaExpr *E) {
       LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
       Sema::ConstraintEvalRAII<TemplateInstantiator> RAII(*this);

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1112,6 +1112,13 @@ TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
     return nullptr;
 
   TypeAliasDecl *Pattern = D->getTemplatedDecl();
+  Sema::InstantiatingTemplate InstTemplate(
+      SemaRef, D->getBeginLoc(), D,
+      D->getTemplateDepth() >= TemplateArgs.getNumLevels()
+          ? ArrayRef<TemplateArgument>()
+          : (TemplateArgs.begin() + TemplateArgs.getNumLevels() - 1 -
+             D->getTemplateDepth())
+                ->Args);
 
   TypeAliasTemplateDecl *PrevAliasTemplate = nullptr;
   if (getPreviousDeclForInstantiation<TypedefNameDecl>(Pattern)) {

clang/lib/Sema/TreeTransform.h

@@ -767,6 +767,12 @@ class TreeTransform {
   /// the body.
   StmtResult SkipLambdaBody(LambdaExpr *E, Stmt *Body);
 
+  CXXRecordDecl::LambdaDependencyKind
+  ComputeLambdaDependency(LambdaScopeInfo *LSI) {
+    return static_cast<CXXRecordDecl::LambdaDependencyKind>(
+        LSI->Lambda->getLambdaDependencyKind());
+  }
+
   QualType TransformReferenceType(TypeLocBuilder &TLB, ReferenceTypeLoc TL);
 
   StmtResult TransformCompoundStmt(CompoundStmt *S, bool IsStmtExpr);
@@ -13936,6 +13942,46 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
                                     /*IsInstantiation*/ true);
   SavedContext.pop();
 
+  // Recompute the dependency of the lambda so that we can defer the lambda call
+  // construction until after we have all the necessary template arguments. For
+  // example, given
+  //
+  // template <class> struct S {
+  //   template <class U>
+  //   using Type = decltype([](U){}(42.0));
+  // };
+  // void foo() {
+  //   using T = S<int>::Type<float>;
+  //             ^~~~~~
+  // }
+  //
+  // We would end up here from instantiating S<int> when ensuring its
+  // completeness. That would transform the lambda call expression regardless of
+  // the absence of the corresponding argument for U.
+  //
+  // Going ahead with unsubstituted type U makes things worse: we would soon
+  // compare the argument type (which is float) against the parameter U
+  // somewhere in Sema::BuildCallExpr. Then we would quickly run into a bogus
+  // error suggesting unmatched types 'U' and 'float'!
+  //
+  // That said, everything will be fine if we defer that semantic checking.
+  // Fortunately, we have such a mechanism that bypasses it if the CallExpr is
+  // dependent. Since the CallExpr's dependency boils down to the lambda's
+  // dependency in this case, we can harness that by recomputing the dependency
+  // from the instantiation arguments.
+  //
+  // FIXME: Creating the type of a lambda requires us to have a dependency
+  // value, which happens before its substitution. We update its dependency
+  // *after* the substitution in case we can't decide the dependency
+  // so early, e.g. because we want to see if any of the *substituted*
+  // parameters are dependent.
+  DependencyKind = getDerived().ComputeLambdaDependency(&LSICopy);
+  Class->setLambdaDependencyKind(DependencyKind);
+  // Clean up the type cache created previously. Then, we re-create a type for
+  // such Decl with the new DependencyKind.
+  Class->setTypeForDecl(nullptr);
+  getSema().Context.getTypeDeclType(Class);
+
   return getSema().BuildLambdaExpr(E->getBeginLoc(), Body.get()->getEndLoc(),
                                    &LSICopy);
 }