[c++20] Implement P0428R2 - Familiar template syntax for generic lambdas

Differential Revision: https://reviews.llvm.org/D36527

llvm-svn: 359967

clang/include/clang/AST/DeclCXX.h

@@ -1221,6 +1221,9 @@ public:
   /// lambda.
   TemplateParameterList *getGenericLambdaTemplateParameterList() const;
 
+  /// Retrieve the lambda template parameters that were specified explicitly.
+  ArrayRef<NamedDecl *> getLambdaExplicitTemplateParameters() const;
+
   LambdaCaptureDefault getLambdaCaptureDefault() const {
     assert(isLambda());
     return static_cast<LambdaCaptureDefault>(getLambdaData().CaptureDefault);

clang/include/clang/AST/DeclTemplate.h

@@ -176,6 +176,11 @@ public:
     return SourceRange(TemplateLoc, RAngleLoc);
   }
 
+  void print(raw_ostream &Out, const ASTContext &Context,
+             bool OmitTemplateKW = false) const;
+  void print(raw_ostream &Out, const ASTContext &Context,
+             const PrintingPolicy &Policy, bool OmitTemplateKW = false) const;
+
 public:
   // FIXME: workaround for MSVC 2013; remove when no longer needed
   using FixedSizeStorageOwner = TrailingObjects::FixedSizeStorageOwner;

clang/include/clang/AST/ExprCXX.h

@@ -1899,6 +1899,10 @@ public:
   /// parameter list associated with it, or else return null.
   TemplateParameterList *getTemplateParameterList() const;
 
+  /// Get the template parameters were explicitly specified (as opposed to being
+  /// invented by use of an auto parameter).
+  ArrayRef<NamedDecl *> getExplicitTemplateParameters() const;
+
   /// Whether this is a generic lambda.
   bool isGenericLambda() const { return getTemplateParameterList(); }
 

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -2423,6 +2423,10 @@ DEF_TRAVERSE_STMT(LambdaExpr, {
     TypeLoc TL = S->getCallOperator()->getTypeSourceInfo()->getTypeLoc();
     FunctionProtoTypeLoc Proto = TL.getAsAdjusted<FunctionProtoTypeLoc>();
 
+    for (Decl *D : S->getExplicitTemplateParameters()) {
+      // Visit explicit template parameters.
+      TRY_TO(TraverseDecl(D));
+    }
     if (S->hasExplicitParameters()) {
       // Visit parameters.
       for (unsigned I = 0, N = Proto.getNumParams(); I != N; ++I)

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -886,6 +886,16 @@ def warn_cxx14_compat_constexpr_on_lambda : Warning<
 def ext_constexpr_on_lambda_cxx17 : ExtWarn<
   "'constexpr' on lambda expressions is a C++17 extension">, InGroup<CXX17>;
 
+ // C++2a template lambdas
+ def ext_lambda_template_parameter_list: ExtWarn<
+  "explicit template parameter list for lambdas is a C++2a extension">,
+  InGroup<CXX2a>;
+def warn_cxx17_compat_lambda_template_parameter_list: Warning<
+  "explicit template parameter list for lambdas is incompatible with "
+  "C++ standards before C++2a">, InGroup<CXXPre2aCompat>, DefaultIgnore;
+ def err_lambda_template_parameter_list_empty : Error<
+   "lambda template parameter list cannot be empty">;
+
 // Availability attribute
 def err_expected_version : Error<
   "expected a version of the form 'major[.minor[.subminor]]'">;

clang/include/clang/Parse/Parser.h

@@ -250,7 +250,13 @@ class Parser : public CodeCompletionHandler {
       Depth += D;
       AddedLevels += D;
     }
+    void setAddedDepth(unsigned D) {
+      Depth = Depth - AddedLevels + D;
+      AddedLevels = D;
+    }
+  
     unsigned getDepth() const { return Depth; }
+    unsigned getOriginalDepth() const { return Depth - AddedLevels; }
   };
 
   /// Factory object for creating ParsedAttr objects.

clang/include/clang/Sema/ScopeInfo.h

@@ -816,16 +816,24 @@ public:
   /// each 'auto' parameter, during initial AST construction.
   unsigned AutoTemplateParameterDepth = 0;
 
-  /// Store the list of the auto parameters for a generic lambda.
-  /// If this is a generic lambda, store the list of the auto
-  /// parameters converted into TemplateTypeParmDecls into a vector
-  /// that can be used to construct the generic lambda's template
-  /// parameter list, during initial AST construction.
-  SmallVector<TemplateTypeParmDecl*, 4> AutoTemplateParams;
+  /// The number of parameters in the template parameter list that were
+  /// explicitly specified by the user, as opposed to being invented by use
+  /// of an auto parameter.
+  unsigned NumExplicitTemplateParams = 0;
+
+  /// Source range covering the explicit template parameter list (if it exists).
+  SourceRange ExplicitTemplateParamsRange;
+
+  /// Store the list of the template parameters for a generic lambda.
+  /// If this is a generic lambda, this holds the explicit template parameters
+  /// followed by the auto parameters converted into TemplateTypeParmDecls.
+  /// It can be used to construct the generic lambda's template parameter list
+  /// during initial AST construction.
+  SmallVector<NamedDecl*, 4> TemplateParams;
 
   /// If this is a generic lambda, and the template parameter
-  /// list has been created (from the AutoTemplateParams) then
-  /// store a reference to it (cache it to avoid reconstructing it).
+  /// list has been created (from the TemplateParams) then store
+  /// a reference to it (cache it to avoid reconstructing it).
   TemplateParameterList *GLTemplateParameterList = nullptr;
 
   /// Contains all variable-referring-expressions (i.e. DeclRefExprs
@@ -878,9 +886,9 @@ public:
   }
 
   /// Is this scope known to be for a generic lambda? (This will be false until
-  /// we parse the first 'auto'-typed parameter.
+  /// we parse a template parameter list or the first 'auto'-typed parameter).
   bool isGenericLambda() const {
-    return !AutoTemplateParams.empty() || GLTemplateParameterList;
+    return !TemplateParams.empty() || GLTemplateParameterList;
   }
 
   /// Add a variable that might potentially be captured by the

clang/include/clang/Sema/Sema.h

@@ -5723,6 +5723,12 @@ public:
   /// given lambda.
   void finishLambdaExplicitCaptures(sema::LambdaScopeInfo *LSI);
 
+  /// \brief This is called after parsing the explicit template parameter list
+  /// on a lambda (if it exists) in C++2a.
+  void ActOnLambdaExplicitTemplateParameterList(SourceLocation LAngleLoc,
+                                                ArrayRef<NamedDecl *> TParams,
+                                                SourceLocation RAngleLoc);
+
   /// Introduce the lambda parameters into scope.
   void addLambdaParameters(
       ArrayRef<LambdaIntroducer::LambdaCapture> Captures,

clang/lib/AST/DeclCXX.cpp

@@ -1421,13 +1421,30 @@ void CXXRecordDecl::getCaptureFields(
 
 TemplateParameterList *
 CXXRecordDecl::getGenericLambdaTemplateParameterList() const {
-  if (!isLambda()) return nullptr;
+  if (!isGenericLambda()) return nullptr;
   CXXMethodDecl *CallOp = getLambdaCallOperator();
   if (FunctionTemplateDecl *Tmpl = CallOp->getDescribedFunctionTemplate())
     return Tmpl->getTemplateParameters();
   return nullptr;
 }
 
+ArrayRef<NamedDecl *>
+CXXRecordDecl::getLambdaExplicitTemplateParameters() const {
+  TemplateParameterList *List = getGenericLambdaTemplateParameterList();
+  if (!List)
+    return {};
+
+  assert(std::is_partitioned(List->begin(), List->end(),
+                             [](const NamedDecl *D) { return !D->isImplicit(); })
+         && "Explicit template params should be ordered before implicit ones");
+
+  const auto ExplicitEnd = std::lower_bound(List->begin(), List->end(), false,
+                                            [](const NamedDecl *D, bool) {
+    return !D->isImplicit();
+  });
+  return llvm::makeArrayRef(List->begin(), ExplicitEnd);
+}
+
 Decl *CXXRecordDecl::getLambdaContextDecl() const {
   assert(isLambda() && "Not a lambda closure type!");
   ExternalASTSource *Source = getParentASTContext().getExternalSource();

clang/lib/AST/DeclPrinter.cpp

@@ -15,6 +15,7 @@
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclVisitor.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
@@ -105,7 +106,8 @@ namespace {
     void VisitOMPDeclareMapperDecl(OMPDeclareMapperDecl *D);
     void VisitOMPCapturedExprDecl(OMPCapturedExprDecl *D);
 
-    void printTemplateParameters(const TemplateParameterList *Params);
+    void printTemplateParameters(const TemplateParameterList *Params,
+                                 bool OmitTemplateKW = false);
     void printTemplateArguments(const TemplateArgumentList &Args,
                                 const TemplateParameterList *Params = nullptr);
     void prettyPrintAttributes(Decl *D);
@@ -126,6 +128,18 @@ void Decl::print(raw_ostream &Out, const PrintingPolicy &Policy,
   Printer.Visit(const_cast<Decl*>(this));
 }
 
+void TemplateParameterList::print(raw_ostream &Out, const ASTContext &Context,
+                                  bool OmitTemplateKW) const {
+  print(Out, Context, Context.getPrintingPolicy(), OmitTemplateKW);
+}
+
+void TemplateParameterList::print(raw_ostream &Out, const ASTContext &Context,
+                                  const PrintingPolicy &Policy,
+                                  bool OmitTemplateKW) const {
+  DeclPrinter Printer(Out, Policy, Context);
+  Printer.printTemplateParameters(this, OmitTemplateKW);
+}
+
 static QualType GetBaseType(QualType T) {
   // FIXME: This should be on the Type class!
   QualType BaseType = T;
@@ -1002,16 +1016,24 @@ void DeclPrinter::VisitLinkageSpecDecl(LinkageSpecDecl *D) {
     Visit(*D->decls_begin());
 }
 
-void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params) {
+void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params,
+                                          bool OmitTemplateKW) {
   assert(Params);
 
-  Out << "template <";
+  if (!OmitTemplateKW)
+    Out << "template ";
+  Out << '<';
 
-  for (unsigned i = 0, e = Params->size(); i != e; ++i) {
-    if (i != 0)
+  bool NeedComma = false;
+  for (const Decl *Param : *Params) {
+    if (Param->isImplicit())
+      continue;
+
+    if (NeedComma)
       Out << ", ";
+    else
+      NeedComma = true;
 
-    const Decl *Param = Params->getParam(i);
     if (auto TTP = dyn_cast<TemplateTypeParmDecl>(Param)) {
 
       if (TTP->wasDeclaredWithTypename())
@@ -1021,6 +1043,8 @@ void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params) {
 
       if (TTP->isParameterPack())
         Out << " ...";
+      else if (!TTP->getName().empty())
+        Out << ' ';
 
       Out << *TTP;
 
@@ -1045,7 +1069,9 @@ void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params) {
     }
   }
 
-  Out << "> ";
+  Out << '>';
+  if (!OmitTemplateKW)
+    Out << ' ';
 }
 
 void DeclPrinter::printTemplateArguments(const TemplateArgumentList &Args,

clang/lib/AST/ExprCXX.cpp

@@ -1204,7 +1204,11 @@ CXXMethodDecl *LambdaExpr::getCallOperator() const {
 TemplateParameterList *LambdaExpr::getTemplateParameterList() const {
   CXXRecordDecl *Record = getLambdaClass();
   return Record->getGenericLambdaTemplateParameterList();
+}
 
+ArrayRef<NamedDecl *> LambdaExpr::getExplicitTemplateParameters() const {
+  const CXXRecordDecl *Record = getLambdaClass();
+  return Record->getLambdaExplicitTemplateParameters();
 }
 
 CompoundStmt *LambdaExpr::getBody() const {

clang/lib/AST/ItaniumMangle.cpp

@@ -486,6 +486,7 @@ private:
                        const AbiTagList *AdditionalAbiTags);
   void mangleBlockForPrefix(const BlockDecl *Block);
   void mangleUnqualifiedBlock(const BlockDecl *Block);
+  void mangleTemplateParamDecl(const NamedDecl *Decl);
   void mangleLambda(const CXXRecordDecl *Lambda);
   void mangleNestedName(const NamedDecl *ND, const DeclContext *DC,
                         const AbiTagList *AdditionalAbiTags,
@@ -1372,7 +1373,8 @@ void CXXNameMangler::mangleUnqualifiedName(const NamedDecl *ND,
     // <unnamed-type-name> ::= <closure-type-name>
     //
     // <closure-type-name> ::= Ul <lambda-sig> E [ <nonnegative number> ] _
-    // <lambda-sig> ::= <parameter-type>+   # Parameter types or 'v' for 'void'.
+    // <lambda-sig> ::= <template-param-decl>* <parameter-type>+
+    //     # Parameter types or 'v' for 'void'.
     if (const CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(TD)) {
       if (Record->isLambda() && Record->getLambdaManglingNumber()) {
         assert(!AdditionalAbiTags &&
@@ -1678,6 +1680,24 @@ void CXXNameMangler::mangleUnqualifiedBlock(const BlockDecl *Block) {
   Out << '_';
 }
 
+// <template-param-decl>
+//   ::= Ty                              # template type parameter
+//   ::= Tn <type>                       # template non-type parameter
+//   ::= Tt <template-param-decl>* E     # template template parameter
+void CXXNameMangler::mangleTemplateParamDecl(const NamedDecl *Decl) {
+  if (isa<TemplateTypeParmDecl>(Decl)) {
+    Out << "Ty";
+  } else if (auto *Tn = dyn_cast<NonTypeTemplateParmDecl>(Decl)) {
+    Out << "Tn";
+    mangleType(Tn->getType());
+  } else if (auto *Tt = dyn_cast<TemplateTemplateParmDecl>(Decl)) {
+    Out << "Tt";
+    for (auto *Param : *Tt->getTemplateParameters())
+      mangleTemplateParamDecl(Param);
+    Out << "E";
+  }
+}
+
 void CXXNameMangler::mangleLambda(const CXXRecordDecl *Lambda) {
   // If the context of a closure type is an initializer for a class member
   // (static or nonstatic), it is encoded in a qualified name with a final
@@ -1705,6 +1725,8 @@ void CXXNameMangler::mangleLambda(const CXXRecordDecl *Lambda) {
   }
 
   Out << "Ul";
+  for (auto *D : Lambda->getLambdaExplicitTemplateParameters())
+    mangleTemplateParamDecl(D);
   const FunctionProtoType *Proto = Lambda->getLambdaTypeInfo()->getType()->
                                    getAs<FunctionProtoType>();
   mangleBareFunctionType(Proto, /*MangleReturnType=*/false,

clang/lib/AST/StmtPrinter.cpp

@@ -1900,8 +1900,14 @@ void StmtPrinter::VisitLambdaExpr(LambdaExpr *Node) {
   }
   OS << ']';
 
+  if (!Node->getExplicitTemplateParameters().empty()) {
+    Node->getTemplateParameterList()->print(
+        OS, Node->getLambdaClass()->getASTContext(),
+        /*OmitTemplateKW*/true);
+  }
+
   if (Node->hasExplicitParameters()) {
-    OS << " (";
+    OS << '(';
     CXXMethodDecl *Method = Node->getCallOperator();
     NeedComma = false;
     for (const auto *P : Method->parameters()) {
@@ -1936,9 +1942,8 @@ void StmtPrinter::VisitLambdaExpr(LambdaExpr *Node) {
   }
 
   // Print the body.
-  CompoundStmt *Body = Node->getBody();
   OS << ' ';
-  PrintStmt(Body);
+  PrintRawCompoundStmt(Node->getBody());
 }
 
 void StmtPrinter::VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *Node) {

clang/lib/AST/TypePrinter.cpp

@@ -1217,8 +1217,18 @@ void TypePrinter::printTemplateTypeParmBefore(const TemplateTypeParmType *T,
                                               raw_ostream &OS) {
   if (IdentifierInfo *Id = T->getIdentifier())
     OS << Id->getName();
+  else {
+    bool IsLambdaAutoParam = false;
+    if (auto D = T->getDecl()) {
+      if (auto M = dyn_cast_or_null<CXXMethodDecl>(D->getDeclContext()))
+        IsLambdaAutoParam = D->isImplicit() && M->getParent()->isLambda();
+    }
+
+    if (IsLambdaAutoParam)
+      OS << "auto";
     else
       OS << "type-parameter-" << T->getDepth() << '-' << T->getIndex();
+  }
   spaceBeforePlaceHolder(OS);
 }
 

clang/lib/Parse/ParseExprCXX.cpp

@@ -638,6 +638,8 @@ ExprResult Parser::ParseCXXIdExpression(bool isAddressOfOperand) {
 ///
 ///       lambda-expression:
 ///         lambda-introducer lambda-declarator[opt] compound-statement
+///         lambda-introducer '<' template-parameter-list '>'
+///             lambda-declarator[opt] compound-statement
 ///
 ///       lambda-introducer:
 ///         '[' lambda-capture[opt] ']'
@@ -1121,6 +1123,33 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
               << A.getName()->getName();
   };
 
+  // FIXME: Consider allowing this as an extension for GCC compatibiblity.
+  const bool HasExplicitTemplateParams = Tok.is(tok::less);
+  ParseScope TemplateParamScope(this, Scope::TemplateParamScope,
+                                /*EnteredScope=*/HasExplicitTemplateParams);
+  if (HasExplicitTemplateParams) {
+    Diag(Tok, getLangOpts().CPlusPlus2a
+                  ? diag::warn_cxx17_compat_lambda_template_parameter_list
+                  : diag::ext_lambda_template_parameter_list);
+
+    SmallVector<NamedDecl*, 4> TemplateParams;
+    SourceLocation LAngleLoc, RAngleLoc;
+    if (ParseTemplateParameters(CurTemplateDepthTracker.getDepth(),
+                                TemplateParams, LAngleLoc, RAngleLoc)) {
+      Actions.ActOnLambdaError(LambdaBeginLoc, getCurScope());
+      return ExprError();
+    }
+
+    if (TemplateParams.empty()) {
+      Diag(RAngleLoc,
+           diag::err_lambda_template_parameter_list_empty);
+    } else {
+      Actions.ActOnLambdaExplicitTemplateParameterList(
+          LAngleLoc, TemplateParams, RAngleLoc);
+      ++CurTemplateDepthTracker;
+    }
+  }
+
   TypeResult TrailingReturnType;
   if (Tok.is(tok::l_paren)) {
     ParseScope PrototypeScope(this,
@@ -1137,13 +1166,20 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
     SourceLocation EllipsisLoc;
 
     if (Tok.isNot(tok::r_paren)) {
-      Actions.RecordParsingTemplateParameterDepth(TemplateParameterDepth);
+      Actions.RecordParsingTemplateParameterDepth(
+          CurTemplateDepthTracker.getOriginalDepth());
+
       ParseParameterDeclarationClause(D, Attr, ParamInfo, EllipsisLoc);
+
       // For a generic lambda, each 'auto' within the parameter declaration
       // clause creates a template type parameter, so increment the depth.
+      // If we've parsed any explicit template parameters, then the depth will
+      // have already been incremented. So we make sure that at most a single
+      // depth level is added.
       if (Actions.getCurGenericLambda())
-        ++CurTemplateDepthTracker;
+        CurTemplateDepthTracker.setAddedDepth(1);
     }
+
     T.consumeClose();
     SourceLocation RParenLoc = T.getCloseLocation();
     SourceLocation DeclEndLoc = RParenLoc;
@@ -1298,6 +1334,7 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
 
   StmtResult Stmt(ParseCompoundStatementBody());
   BodyScope.Exit();
+  TemplateParamScope.Exit();
 
   if (!Stmt.isInvalid() && !TrailingReturnType.isInvalid())
     return Actions.ActOnLambdaExpr(LambdaBeginLoc, Stmt.get(), getCurScope());

clang/lib/Sema/Sema.cpp

@@ -1793,7 +1793,7 @@ LambdaScopeInfo *Sema::getCurLambda(bool IgnoreNonLambdaCapturingScope) {
 // an associated template parameter list.
 LambdaScopeInfo *Sema::getCurGenericLambda() {
   if (LambdaScopeInfo *LSI =  getCurLambda()) {
-    return (LSI->AutoTemplateParams.size() ||
+    return (LSI->TemplateParams.size() ||
                     LSI->GLTemplateParameterList) ? LSI : nullptr;
   }
   return nullptr;

clang/lib/Sema/SemaLambda.cpp

@@ -20,6 +20,7 @@
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/SemaLambda.h"
+#include "llvm/ADT/STLExtras.h"
 using namespace clang;
 using namespace sema;
 
@@ -225,19 +226,14 @@ Optional<unsigned> clang::getStackIndexOfNearestEnclosingCaptureCapableLambda(
 
 static inline TemplateParameterList *
 getGenericLambdaTemplateParameterList(LambdaScopeInfo *LSI, Sema &SemaRef) {
-  if (LSI->GLTemplateParameterList)
-    return LSI->GLTemplateParameterList;
-
-  if (!LSI->AutoTemplateParams.empty()) {
-    SourceRange IntroRange = LSI->IntroducerRange;
-    SourceLocation LAngleLoc = IntroRange.getBegin();
-    SourceLocation RAngleLoc = IntroRange.getEnd();
+  if (!LSI->GLTemplateParameterList && !LSI->TemplateParams.empty()) {
     LSI->GLTemplateParameterList = TemplateParameterList::Create(
         SemaRef.Context,
-        /*Template kw loc*/ SourceLocation(), LAngleLoc,
-        llvm::makeArrayRef((NamedDecl *const *)LSI->AutoTemplateParams.data(),
-                           LSI->AutoTemplateParams.size()),
-        RAngleLoc, nullptr);
+        /*Template kw loc*/ SourceLocation(),
+        /*L angle loc*/ LSI->ExplicitTemplateParamsRange.getBegin(),
+        LSI->TemplateParams,
+        /*R angle loc*/LSI->ExplicitTemplateParamsRange.getEnd(),
+        nullptr);
   }
   return LSI->GLTemplateParameterList;
 }
@@ -492,6 +488,23 @@ void Sema::finishLambdaExplicitCaptures(LambdaScopeInfo *LSI) {
   LSI->finishedExplicitCaptures();
 }
 
+void Sema::ActOnLambdaExplicitTemplateParameterList(SourceLocation LAngleLoc,
+                                                    ArrayRef<NamedDecl *> TParams,
+                                                    SourceLocation RAngleLoc) {
+  LambdaScopeInfo *LSI = getCurLambda();
+  assert(LSI && "Expected a lambda scope");
+  assert(LSI->NumExplicitTemplateParams == 0 &&
+         "Already acted on explicit template parameters");
+  assert(LSI->TemplateParams.empty() &&
+         "Explicit template parameters should come "
+         "before invented (auto) ones");
+  assert(!TParams.empty() &&
+         "No template parameters to act on");
+  LSI->TemplateParams.append(TParams.begin(), TParams.end());
+  LSI->NumExplicitTemplateParams = TParams.size();
+  LSI->ExplicitTemplateParamsRange = {LAngleLoc, RAngleLoc};
+}
+
 void Sema::addLambdaParameters(
     ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
     CXXMethodDecl *CallOperator, Scope *CurScope) {
@@ -832,17 +845,23 @@ FieldDecl *Sema::buildInitCaptureField(LambdaScopeInfo *LSI, VarDecl *Var) {
 void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
                                         Declarator &ParamInfo,
                                         Scope *CurScope) {
-  // Determine if we're within a context where we know that the lambda will
-  // be dependent, because there are template parameters in scope.
-  bool KnownDependent = false;
   LambdaScopeInfo *const LSI = getCurLambda();
   assert(LSI && "LambdaScopeInfo should be on stack!");
 
-  // The lambda-expression's closure type might be dependent even if its
-  // semantic context isn't, if it appears within a default argument of a
-  // function template.
-  if (CurScope->getTemplateParamParent())
-    KnownDependent = true;
+  // Determine if we're within a context where we know that the lambda will
+  // be dependent, because there are template parameters in scope.
+  bool KnownDependent;
+  if (LSI->NumExplicitTemplateParams > 0) {
+    auto *TemplateParamScope = CurScope->getTemplateParamParent();
+    assert(TemplateParamScope &&
+           "Lambda with explicit template param list should establish a "
+           "template param scope");
+    assert(TemplateParamScope->getParent());
+    KnownDependent = TemplateParamScope->getParent()
+                                       ->getTemplateParamParent() != nullptr;
+  } else {
+    KnownDependent = CurScope->getTemplateParamParent() != nullptr;
+  }
 
   // Determine the signature of the call operator.
   TypeSourceInfo *MethodTyInfo;

clang/lib/Sema/SemaType.cpp

@@ -2935,7 +2935,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
         sema::LambdaScopeInfo *LSI = SemaRef.getCurLambda();
         assert(LSI && "No LambdaScopeInfo on the stack!");
         const unsigned TemplateParameterDepth = LSI->AutoTemplateParameterDepth;
-        const unsigned AutoParameterPosition = LSI->AutoTemplateParams.size();
+        const unsigned AutoParameterPosition = LSI->TemplateParams.size();
         const bool IsParameterPack = D.hasEllipsis();
 
         // Create the TemplateTypeParmDecl here to retrieve the corresponding
@@ -2947,7 +2947,8 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
                 /*KeyLoc*/ SourceLocation(), /*NameLoc*/ D.getBeginLoc(),
                 TemplateParameterDepth, AutoParameterPosition,
                 /*Identifier*/ nullptr, false, IsParameterPack);
-        LSI->AutoTemplateParams.push_back(CorrespondingTemplateParam);
+        CorrespondingTemplateParam->setImplicit();
+        LSI->TemplateParams.push_back(CorrespondingTemplateParam);
         // Replace the 'auto' in the function parameter with this invented
         // template type parameter.
         // FIXME: Retain some type sugar to indicate that this was written

clang/test/CXX/temp/temp.decls/temp.variadic/p4.cpp

@@ -213,8 +213,10 @@ namespace PackExpansionWithinLambda {
       };
 #endif
 
+#if __cplusplus > 201703L
       //    - in a template parameter pack that is a pack expansion
-      // FIXME: We do not support any way to reach this case yet.
+      swallow([]<T *...v, template<T *> typename ...W>(W<v> ...wv) { });
+#endif
 
       //    - in an initializer-list
       int arr[] = {T().x...};
@@ -279,11 +281,6 @@ namespace PackExpansionWithinLambda {
   struct T { int x; using U = int; };
   void g() { f<T>(1, 2, 3); }
 
-  template<typename ...T, typename ...U> void pack_in_lambda(U ...u) { // expected-note {{here}}
-    // FIXME: Move this test into 'f' above once we support this syntax.
-    []<T *...v, template<T *> typename ...U>(U<v> ...uv) {}; // expected-error {{expected body of lambda}} expected-error {{does not refer to a value}}
-  }
-
   template<typename ...T> void pack_expand_attr() {
     // FIXME: Move this test into 'f' above once we support this.
     [[gnu::aligned(alignof(T))...]] int x; // expected-error {{cannot be used as an attribute pack}} expected-error {{unexpanded}}

clang/test/CodeGenCXX/mangle-lambda-explicit-template-params.cpp

@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++2a -triple %itanium_abi_triple -emit-llvm -o - %s -w | FileCheck %s
+
+template<class, int, class>
+struct DummyType { };
+
+inline void inline_func() {
+  // CHECK: UlvE
+  []{}();
+
+  // CHECK: UlTyvE
+  []<class>{}.operator()<int>();
+
+  // CHECK: UlTyT_E
+  []<class T>(T){}(1);
+
+  // CHECK: UlTyTyT_T0_E
+  []<class T1, class T2>(T1, T2){}(1, 2);
+
+  // CHECK: UlTyTyT0_T_E
+  []<class T1, class T2>(T2, T1){}(2, 1);
+
+  // CHECK: UlTniTyTnjT0_E
+  []<int I, class T, unsigned U>(T){}.operator()<1, int, 2>(3);
+
+  // CHECK: UlTyTtTyTniTyETniTyvE
+  []<class,
+     template<class, int, class> class,
+     int,
+     class>{}.operator()<unsigned, DummyType, 5, int>();
+}
+
+void call_inline_func() {
+  inline_func();
+}

clang/test/PCH/cxx1y-lambdas.mm

@@ -50,7 +50,7 @@ int add(int x, int y) {
 }
 
 // CHECK-PRINT: inline int add_int_slowly_twice 
-// CHECK-PRINT: lambda = [] (type-parameter-0-0 z
+// CHECK-PRINT: lambda = [](auto z
 
 // CHECK-PRINT: init_capture
 // CHECK-PRINT: [&, x(t)]

clang/test/PCH/cxx2a-template-lambdas.cpp

@@ -0,0 +1,42 @@
+// RUN: %clang_cc1 -std=c++2a -emit-pch %s -o %t
+// RUN: %clang_cc1 -std=c++2a -include-pch %t -verify %s
+
+// expected-no-diagnostics
+
+#ifndef HEADER
+#define HEADER
+
+auto l1 = []<int I>() constexpr -> int {
+    return I;
+};
+
+auto l2 = []<auto I>() constexpr -> decltype(I) {
+    return I;
+};
+
+auto l3 = []<class T>(auto i) constexpr -> T {
+  return T(i);
+};
+
+auto l4 = []<template<class> class T, class U>(T<U>, auto i) constexpr -> U {
+  return U(i);
+};
+
+#else /*included pch*/
+
+static_assert(l1.operator()<5>() == 5);
+static_assert(l1.operator()<6>() == 6);
+
+static_assert(l2.operator()<7>() == 7);
+static_assert(l2.operator()<nullptr>() == nullptr);
+
+static_assert(l3.operator()<int>(8.4) == 8);
+static_assert(l3.operator()<int>(9.9) == 9);
+
+template<typename T>
+struct DummyTemplate { };
+
+static_assert(l4(DummyTemplate<float>(), 12) == 12.0);
+static_assert(l4(DummyTemplate<int>(), 19.8) == 19);
+
+#endif // HEADER

clang/test/Parser/cxx2a-template-lambdas.cpp

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -std=c++2a %s -verify
+
+auto L0 = []<> { }; //expected-error {{cannot be empty}}
+
+auto L1 = []<typename T1, typename T2> { };
+auto L2 = []<typename T1, typename T2>(T1 arg1, T2 arg2) -> T1 { };
+auto L3 = []<typename T>(auto arg) { T t; };
+auto L4 = []<int I>() { };

clang/test/SemaCXX/cxx2a-template-lambdas.cpp

@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -std=c++2a -verify %s
+
+template<typename, typename>
+constexpr bool is_same = false;
+
+template<typename T>
+constexpr bool is_same<T, T> = true;
+
+template<typename T>
+struct DummyTemplate { };
+
+void func() {
+  auto L0 = []<typename T>(T arg) {
+    static_assert(is_same<T, int>); // expected-error {{static_assert failed}}
+  };
+  L0(0);
+  L0(0.0); // expected-note {{in instantiation}}
+
+  auto L1 = []<int I> {
+    static_assert(I == 5); // expected-error {{static_assert failed}}
+  };
+  L1.operator()<5>();
+  L1.operator()<6>(); // expected-note {{in instantiation}}
+
+  auto L2 = []<template<typename> class T, class U>(T<U> &&arg) {
+    static_assert(is_same<T<U>, DummyTemplate<float>>); // // expected-error {{static_assert failed}}
+  };
+  L2(DummyTemplate<float>());
+  L2(DummyTemplate<double>()); // expected-note {{in instantiation}}
+}
+
+template<typename T> // expected-note {{declared here}}
+struct ShadowMe {
+  void member_func() {
+    auto L = []<typename T> { }; // expected-error {{'T' shadows template parameter}}
+  }
+};
+
+template<typename T>
+constexpr T outer() {
+  return []<T x>() { return x; }.template operator()<123>(); // expected-error {{no matching member function}} \
+                                                                expected-note {{candidate template ignored}}
+}
+static_assert(outer<int>() == 123);
+template int *outer<int *>(); // expected-note {{in instantiation}}

clang/unittests/AST/StmtPrinterTest.cpp

@@ -157,6 +157,43 @@ TEST(StmtPrinter, TestCXXConversionDeclExplicit) {
     // WRONG; Should be: (a & b).operator void *()
 }
 
+TEST(StmtPrinter, TestCXXLamda) {
+  ASSERT_TRUE(PrintedStmtCXXMatches(StdVer::CXX11,
+    "void A() {"
+    "  auto l = [] { };"
+    "}",
+    lambdaExpr(anything()).bind("id"),
+    "[] {\n"
+    "}"));
+
+  ASSERT_TRUE(PrintedStmtCXXMatches(StdVer::CXX11,
+    "void A() {"
+    "  int a = 0, b = 1;"
+    "  auto l = [a,b](int c, float d) { };"
+    "}",
+    lambdaExpr(anything()).bind("id"),
+    "[a, b](int c, float d) {\n"
+    "}"));
+
+  ASSERT_TRUE(PrintedStmtCXXMatches(StdVer::CXX14,
+    "void A() {"
+    "  auto l = [](auto a, int b, auto c, int, auto) { };"
+    "}",
+    lambdaExpr(anything()).bind("id"),
+    "[](auto a, int b, auto c, int, auto) {\n"
+    "}"));
+
+  ASSERT_TRUE(PrintedStmtCXXMatches(StdVer::CXX2a,
+    "void A() {"
+    "  auto l = []<typename T1, class T2, int I,"
+    "              template<class, typename> class T3>"
+    "           (int a, auto, int, auto d) { };"
+    "}",
+    lambdaExpr(anything()).bind("id"),
+    "[]<typename T1, class T2, int I, template <class, typename> class T3>(int a, auto, int, auto d) {\n"
+    "}"));
+}
+
 TEST(StmtPrinter, TestNoImplicitBases) {
   const char *CPPSource = R"(
 class A {

clang/unittests/Tooling/RecursiveASTVisitorTests/LambdaTemplateParams.cpp

@@ -0,0 +1,53 @@
+//===- unittest/Tooling/RecursiveASTVisitorTests/LambdaTemplateParams.cpp -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "TestVisitor.h"
+
+using namespace clang;
+
+namespace {
+
+// Matches (optional) explicit template parameters.
+class LambdaTemplateParametersVisitor
+  : public ExpectedLocationVisitor<LambdaTemplateParametersVisitor> {
+public:
+  bool shouldVisitImplicitCode() const { return false; }
+
+  bool VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
+    EXPECT_FALSE(D->isImplicit());
+    Match(D->getName(), D->getLocStart());
+    return true;
+  }
+
+  bool VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
+    EXPECT_FALSE(D->isImplicit());
+    Match(D->getName(), D->getLocStart());
+    return true;
+  }
+
+  bool VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D) {
+    EXPECT_FALSE(D->isImplicit());
+    Match(D->getName(), D->getLocStart());
+    return true;
+  }
+};
+
+TEST(RecursiveASTVisitor, VisitsLambdaExplicitTemplateParameters) {
+  LambdaTemplateParametersVisitor Visitor;
+  Visitor.ExpectMatch("T",  2, 15);
+  Visitor.ExpectMatch("I",  2, 24);
+  Visitor.ExpectMatch("TT", 2, 31);
+  EXPECT_TRUE(Visitor.runOver(
+      "void f() { \n"
+      "  auto l = []<class T, int I, template<class> class TT>(auto p) { }; \n"
+      "}",
+      LambdaTemplateParametersVisitor::Lang_CXX2a));
+}
+
+} // end anonymous namespace

clang/www/cxx_status.html

@@ -868,7 +868,7 @@ as the draft C++2a standard evolves.
     <tr>
       <td><i>template-parameter-list</i> for generic lambdas</td>
       <td><a href="http://wg21.link/p0428r2">P0428R2</a></td>
-      <td class="none" align="center">No</td>
+      <td class="svn" align="center">SVN</td>
     </tr>
     <tr id="p0734">
       <td rowspan="4">Concepts</td>