[Clang] Implement Change scope of lambda trailing-return-type

This implements P2036R3 and P2579R0.
That is, explicit, int, and implicit capture become visible
at the start of the parameter head.

Reviewed By: aaron.ballman, rupprecht, shafik

Differential Revision: https://reviews.llvm.org/D124351

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -82,6 +82,11 @@ C++20 Feature Support
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
+- Implemented `P2036R3: Change scope of lambda trailing-return-type <https://wg21.link/P2036R3>`_
+  and `P2579R0 Mitigation strategies for P2036 <https://wg21.link/P2579R0>`_.
+  These proposals modify how variables captured in lambdas can appear in trailing return type
+  expressions and how their types are deduced therein, in all C++ language versions.
+
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/DeclCXX.h

@@ -1092,6 +1092,11 @@ class CXXRecordDecl : public RecordDecl {
 
   unsigned capture_size() const { return getLambdaData().NumCaptures; }
 
+  const LambdaCapture *getCapture(unsigned I) const {
+    assert(isLambda() && I < capture_size() && "invalid index for capture");
+    return captures_begin() + I;
+  }
+
   using conversion_iterator = UnresolvedSetIterator;
 
   conversion_iterator conversion_begin() const {
@@ -1826,6 +1831,20 @@ class CXXRecordDecl : public RecordDecl {
     return getLambdaData().MethodTyInfo;
   }
 
+  void setLambdaTypeInfo(TypeSourceInfo *TS) {
+    assert(DefinitionData && DefinitionData->IsLambda &&
+           "setting lambda property of non-lambda class");
+    auto &DL = static_cast<LambdaDefinitionData &>(*DefinitionData);
+    DL.MethodTyInfo = TS;
+  }
+
+  void setLambdaIsGeneric(bool IsGeneric) {
+    assert(DefinitionData && DefinitionData->IsLambda &&
+           "setting lambda property of non-lambda class");
+    auto &DL = static_cast<LambdaDefinitionData &>(*DefinitionData);
+    DL.IsGenericLambda = IsGeneric;
+  }
+
   // Determine whether this type is an Interface Like type for
   // __interface inheritance purposes.
   bool isInterfaceLike() const;

clang/include/clang/Sema/Scope.h

@@ -145,6 +145,11 @@ class Scope {
     /// This is a scope of some OpenMP directive with
     /// order clause which specifies concurrent
     OpenMPOrderClauseScope = 0x4000000,
+    /// This is the scope for a lambda, after the lambda introducer.
+    /// Lambdas need two FunctionPrototypeScope scopes (because there is a
+    /// template scope in between), the outer scope does not increase the
+    /// depth of recursion.
+    LambdaScope = 0x8000000,
   };
 
 private:

clang/include/clang/Sema/ScopeInfo.h

@@ -838,6 +838,11 @@ class LambdaScopeInfo final :
   /// The lambda's compiler-generated \c operator().
   CXXMethodDecl *CallOperator = nullptr;
 
+  /// Indicate that we parsed the parameter list
+  /// at which point the mutability of the lambda
+  /// is known.
+  bool AfterParameterList = true;
+
   /// Source range covering the lambda introducer [...].
   SourceRange IntroducerRange;
 
@@ -849,8 +854,9 @@ class LambdaScopeInfo final :
   /// explicit captures.
   unsigned NumExplicitCaptures = 0;
 
-  /// Whether this is a mutable lambda.
-  bool Mutable = false;
+  /// Whether this is a mutable lambda. Until the mutable keyword is parsed,
+  /// we assume the lambda is mutable.
+  bool Mutable = true;
 
   /// Whether the (empty) parameter list is explicit.
   bool ExplicitParams = false;

clang/include/clang/Sema/Sema.h

@@ -7089,15 +7089,21 @@ class Sema final {
           std::nullopt);
 
   /// Endow the lambda scope info with the relevant properties.
-  void buildLambdaScope(sema::LambdaScopeInfo *LSI,
-                        CXXMethodDecl *CallOperator,
+  void buildLambdaScope(sema::LambdaScopeInfo *LSI, CXXMethodDecl *CallOperator,
                         SourceRange IntroducerRange,
                         LambdaCaptureDefault CaptureDefault,
-                        SourceLocation CaptureDefaultLoc,
-                        bool ExplicitParams,
-                        bool ExplicitResultType,
+                        SourceLocation CaptureDefaultLoc, bool ExplicitParams,
                         bool Mutable);
 
+  CXXMethodDecl *CreateLambdaCallOperator(SourceRange IntroducerRange,
+                                          CXXRecordDecl *Class);
+  void CompleteLambdaCallOperator(
+      CXXMethodDecl *Method, SourceLocation LambdaLoc,
+      SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
+      TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
+      StorageClass SC, ArrayRef<ParmVarDecl *> Params,
+      bool HasExplicitResultType);
+
   /// Perform initialization analysis of the init-capture and perform
   /// any implicit conversions such as an lvalue-to-rvalue conversion if
   /// not being used to initialize a reference.
@@ -7118,11 +7124,9 @@ class Sema final {
   ///
   ///  CodeGen handles emission of lambda captures, ignoring these dummy
   ///  variables appropriately.
-  VarDecl *createLambdaInitCaptureVarDecl(SourceLocation Loc,
-                                          QualType InitCaptureType,
-                                          SourceLocation EllipsisLoc,
-                                          IdentifierInfo *Id,
-                                          unsigned InitStyle, Expr *Init);
+  VarDecl *createLambdaInitCaptureVarDecl(
+      SourceLocation Loc, QualType InitCaptureType, SourceLocation EllipsisLoc,
+      IdentifierInfo *Id, unsigned InitStyle, Expr *Init, DeclContext *DeclCtx);
 
   /// Add an init-capture to a lambda scope.
   void addInitCapture(sema::LambdaScopeInfo *LSI, VarDecl *Var,
@@ -7132,28 +7136,38 @@ class Sema final {
   /// given lambda.
   void finishLambdaExplicitCaptures(sema::LambdaScopeInfo *LSI);
 
-  /// \brief This is called after parsing the explicit template parameter list
+  /// Deduce a block or lambda's return type based on the return
+  /// statements present in the body.
+  void deduceClosureReturnType(sema::CapturingScopeInfo &CSI);
+
+  /// Once the Lambdas capture are known, we can start to create the closure,
+  /// call operator method, and keep track of the captures.
+  /// We do the capture lookup here, but they are not actually captured until
+  /// after we know what the qualifiers of the call operator are.
+  void ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
+                                            Scope *CurContext);
+
+  /// This is called after parsing the explicit template parameter list
   /// on a lambda (if it exists) in C++2a.
-  void ActOnLambdaExplicitTemplateParameterList(SourceLocation LAngleLoc,
+  void ActOnLambdaExplicitTemplateParameterList(LambdaIntroducer &Intro,
+                                                SourceLocation LAngleLoc,
                                                 ArrayRef<NamedDecl *> TParams,
                                                 SourceLocation RAngleLoc,
                                                 ExprResult RequiresClause);
 
-  /// Introduce the lambda parameters into scope.
-  void addLambdaParameters(
-      ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
-      CXXMethodDecl *CallOperator, Scope *CurScope);
+  void ActOnLambdaClosureQualifiers(LambdaIntroducer &Intro,
+                                    SourceLocation MutableLoc);
 
-  /// Deduce a block or lambda's return type based on the return
-  /// statements present in the body.
-  void deduceClosureReturnType(sema::CapturingScopeInfo &CSI);
+  void ActOnLambdaClosureParameters(
+      Scope *LambdaScope,
+      MutableArrayRef<DeclaratorChunk::ParamInfo> ParamInfo);
 
   /// ActOnStartOfLambdaDefinition - This is called just before we start
   /// parsing the body of a lambda; it analyzes the explicit captures and
   /// arguments, and sets up various data-structures for the body of the
   /// lambda.
   void ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
-                                    Declarator &ParamInfo, Scope *CurScope);
+                                    Declarator &ParamInfo, const DeclSpec &DS);
 
   /// ActOnLambdaError - If there is an error parsing a lambda, this callback
   /// is invoked to pop the information about the lambda.
@@ -7248,6 +7262,13 @@ class Sema final {
       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);
+
   /// used by SetupConstraintCheckingTemplateArgumentsAndScope to recursively(in
   /// the case of lambdas) set up the LocalInstantiationScope of the current
   /// function.