Implement CWG2631

Implement https://cplusplus.github.io/CWG/issues/2631.html.

Immediate calls in default arguments and defaults members
are not evaluated.

Instead, we evaluate them when constructing a
`CXXDefaultArgExpr`/`BuildCXXDefaultInitExpr`.

The immediate calls are executed by doing a
transform on the initializing expression.

Note that lambdas are not considering subexpressions so
we do not need to transform them.

As a result of this patch, unused default member
initializers are not considered odr-used, and
errors about members binding to local variables
in an outer scope only surface at the point
where a constructor is defined.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D136554

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -645,6 +645,11 @@ C++ Language Changes in Clang
 - Implemented DR2358 allowing init captures in lambdas in default arguments.
 - implemented `DR2654 <https://wg21.link/cwg2654>`_ which undeprecates
   all compound assignements operations on volatile qualified variables.
+- Implemented DR2631. Invalid ``consteval`` calls in default arguments and default
+  member initializers are diagnosed when and if the default is used.
+  This Fixes `Issue 56379 <https://github.com/llvm/llvm-project/issues/56379>`_
+  and changes the value of ``std::source_location::current()``
+  used in default parameters calls compared to previous versions of Clang.
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ExprCXX.h

@@ -1244,8 +1244,12 @@ class CXXThrowExpr : public Expr {
 /// This wraps up a function call argument that was created from the
 /// corresponding parameter's default argument, when the call did not
 /// explicitly supply arguments for all of the parameters.
-class CXXDefaultArgExpr final : public Expr {
+class CXXDefaultArgExpr final
+    : public Expr,
+      private llvm::TrailingObjects<CXXDefaultArgExpr, Expr *> {
   friend class ASTStmtReader;
+  friend class ASTReader;
+  friend TrailingObjects;
 
   /// The parameter whose default is being used.
   ParmVarDecl *Param;
@@ -1254,7 +1258,7 @@ class CXXDefaultArgExpr final : public Expr {
   DeclContext *UsedContext;
 
   CXXDefaultArgExpr(StmtClass SC, SourceLocation Loc, ParmVarDecl *Param,
-                    DeclContext *UsedContext)
+                    Expr *RewrittenExpr, DeclContext *UsedContext)
       : Expr(SC,
              Param->hasUnparsedDefaultArg()
                  ? Param->getType().getNonReferenceType()
@@ -1263,28 +1267,58 @@ class CXXDefaultArgExpr final : public Expr {
              Param->getDefaultArg()->getObjectKind()),
         Param(Param), UsedContext(UsedContext) {
     CXXDefaultArgExprBits.Loc = Loc;
+    CXXDefaultArgExprBits.HasRewrittenInit = RewrittenExpr != nullptr;
+    if (RewrittenExpr)
+      *getTrailingObjects<Expr *>() = RewrittenExpr;
     setDependence(computeDependence(this));
   }
 
+  CXXDefaultArgExpr(EmptyShell Empty, bool HasRewrittenInit)
+      : Expr(CXXDefaultArgExprClass, Empty) {
+    CXXDefaultArgExprBits.HasRewrittenInit = HasRewrittenInit;
+  }
+
+  size_t numTrailingObjects() const {
+    return CXXDefaultArgExprBits.HasRewrittenInit;
+  }
+
 public:
-  CXXDefaultArgExpr(EmptyShell Empty) : Expr(CXXDefaultArgExprClass, Empty) {}
+  static CXXDefaultArgExpr *CreateEmpty(const ASTContext &C,
+                                        bool HasRewrittenInit);
 
   // \p Param is the parameter whose default argument is used by this
   // expression.
   static CXXDefaultArgExpr *Create(const ASTContext &C, SourceLocation Loc,
-                                   ParmVarDecl *Param,
-                                   DeclContext *UsedContext) {
-    return new (C)
-        CXXDefaultArgExpr(CXXDefaultArgExprClass, Loc, Param, UsedContext);
-  }
-
+                                   ParmVarDecl *Param, Expr *RewrittenExpr,
+                                   DeclContext *UsedContext);
   // Retrieve the parameter that the argument was created from.
   const ParmVarDecl *getParam() const { return Param; }
   ParmVarDecl *getParam() { return Param; }
 
-  // Retrieve the actual argument to the function call.
-  const Expr *getExpr() const { return getParam()->getDefaultArg(); }
-  Expr *getExpr() { return getParam()->getDefaultArg(); }
+  bool hasRewrittenInit() const {
+    return CXXDefaultArgExprBits.HasRewrittenInit;
+  }
+
+  // Retrieve the argument to the function call.
+  Expr *getExpr();
+  const Expr *getExpr() const {
+    return const_cast<CXXDefaultArgExpr *>(this)->getExpr();
+  }
+
+  Expr *getRewrittenExpr() {
+    return hasRewrittenInit() ? *getTrailingObjects<Expr *>() : nullptr;
+  }
+
+  const Expr *getRewrittenExpr() const {
+    return const_cast<CXXDefaultArgExpr *>(this)->getRewrittenExpr();
+  }
+
+  // Retrieve the rewritten init expression (for an init expression containing
+  // immediate calls) with the top level FullExpr and ConstantExpr stripped off.
+  Expr *getAdjustedRewrittenExpr();
+  const Expr *getAdjustedRewrittenExpr() const {
+    return const_cast<CXXDefaultArgExpr *>(this)->getAdjustedRewrittenExpr();
+  }
 
   const DeclContext *getUsedContext() const { return UsedContext; }
   DeclContext *getUsedContext() { return UsedContext; }
@@ -1321,41 +1355,67 @@ class CXXDefaultArgExpr final : public Expr {
 /// is implicitly used in a mem-initializer-list in a constructor
 /// (C++11 [class.base.init]p8) or in aggregate initialization
 /// (C++1y [dcl.init.aggr]p7).
-class CXXDefaultInitExpr : public Expr {
-  friend class ASTReader;
-  friend class ASTStmtReader;
+class CXXDefaultInitExpr final
+    : public Expr,
+      private llvm::TrailingObjects<CXXDefaultArgExpr, Expr *> {
 
+  friend class ASTStmtReader;
+  friend class ASTReader;
+  friend TrailingObjects;
   /// The field whose default is being used.
   FieldDecl *Field;
 
   /// The context where the default initializer expression was used.
   DeclContext *UsedContext;
 
   CXXDefaultInitExpr(const ASTContext &Ctx, SourceLocation Loc,
-                     FieldDecl *Field, QualType Ty, DeclContext *UsedContext);
+                     FieldDecl *Field, QualType Ty, DeclContext *UsedContext,
+                     Expr *RewrittenInitExpr);
+
+  CXXDefaultInitExpr(EmptyShell Empty, bool HasRewrittenInit)
+      : Expr(CXXDefaultInitExprClass, Empty) {
+    CXXDefaultInitExprBits.HasRewrittenInit = HasRewrittenInit;
+  }
 
-  CXXDefaultInitExpr(EmptyShell Empty) : Expr(CXXDefaultInitExprClass, Empty) {}
+  size_t numTrailingObjects() const {
+    return CXXDefaultInitExprBits.HasRewrittenInit;
+  }
 
 public:
+  static CXXDefaultInitExpr *CreateEmpty(const ASTContext &C,
+                                         bool HasRewrittenInit);
   /// \p Field is the non-static data member whose default initializer is used
   /// by this expression.
   static CXXDefaultInitExpr *Create(const ASTContext &Ctx, SourceLocation Loc,
-                                    FieldDecl *Field, DeclContext *UsedContext) {
-    return new (Ctx) CXXDefaultInitExpr(Ctx, Loc, Field, Field->getType(), UsedContext);
+                                    FieldDecl *Field, DeclContext *UsedContext,
+                                    Expr *RewrittenInitExpr);
+
+  bool hasRewrittenInit() const {
+    return CXXDefaultInitExprBits.HasRewrittenInit;
   }
 
   /// Get the field whose initializer will be used.
   FieldDecl *getField() { return Field; }
   const FieldDecl *getField() const { return Field; }
 
   /// Get the initialization expression that will be used.
+  Expr *getExpr();
   const Expr *getExpr() const {
-    assert(Field->getInClassInitializer() && "initializer hasn't been parsed");
-    return Field->getInClassInitializer();
+    return const_cast<CXXDefaultInitExpr *>(this)->getExpr();
+  }
+
+  /// Retrieve the initializing expression with evaluated immediate calls, if
+  /// any.
+  const Expr *getRewrittenExpr() const {
+    assert(hasRewrittenInit() && "expected a rewritten init expression");
+    return *getTrailingObjects<Expr *>();
   }
-  Expr *getExpr() {
-    assert(Field->getInClassInitializer() && "initializer hasn't been parsed");
-    return Field->getInClassInitializer();
+
+  /// Retrieve the initializing expression with evaluated immediate calls, if
+  /// any.
+  Expr *getRewrittenExpr() {
+    assert(hasRewrittenInit() && "expected a rewritten init expression");
+    return *getTrailingObjects<Expr *>();
   }
 
   const DeclContext *getUsedContext() const { return UsedContext; }

clang/include/clang/AST/Stmt.h

@@ -690,6 +690,9 @@ class alignas(void *) Stmt {
 
     unsigned : NumExprBits;
 
+    /// Whether this CXXDefaultArgExpr rewrote its argument and stores a copy.
+    unsigned HasRewrittenInit : 1;
+
     /// The location where the default argument expression was used.
     SourceLocation Loc;
   };
@@ -700,6 +703,10 @@ class alignas(void *) Stmt {
 
     unsigned : NumExprBits;
 
+    /// Whether this CXXDefaultInitExprBitfields rewrote its argument and stores
+    /// a copy.
+    unsigned HasRewrittenInit : 1;
+
     /// The location where the default initializer expression was used.
     SourceLocation Loc;
   };

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2646,6 +2646,10 @@ def err_invalid_consteval_take_address : Error<
   " of an immediate invocation">;
 def err_invalid_consteval_call : Error<
   "call to consteval function %q0 is not a constant expression">;
+def note_invalid_consteval_initializer : Note<
+  "in the default initalizer of %0">;
+def note_invalid_consteval_initializer_here : Note<
+  "initialized here %0">;
 def err_invalid_consteval_decl_kind : Error<
   "%0 cannot be declared consteval">;
 def err_invalid_constexpr : Error<

clang/include/clang/Sema/Sema.h

@@ -1330,6 +1330,25 @@ class Sema final {
     bool InDiscardedStatement;
     bool InImmediateFunctionContext;
 
+    bool IsCurrentlyCheckingDefaultArgumentOrInitializer = false;
+
+    // When evaluating immediate functions in the initializer of a default
+    // argument or default member initializer, this is the declaration whose
+    // default initializer is being evaluated and the location of the call
+    // or constructor definition.
+    struct InitializationContext {
+      InitializationContext(SourceLocation Loc, ValueDecl *Decl,
+                            DeclContext *Context)
+          : Loc(Loc), Decl(Decl), Context(Context) {
+        assert(Decl && Context && "invalid initialization context");
+      }
+
+      SourceLocation Loc;
+      ValueDecl *Decl = nullptr;
+      DeclContext *Context = nullptr;
+    };
+    llvm::Optional<InitializationContext> DelayedDefaultInitializationContext;
+
     ExpressionEvaluationContextRecord(ExpressionEvaluationContext Context,
                                       unsigned NumCleanupObjects,
                                       CleanupInfo ParentCleanup,
@@ -6208,19 +6227,22 @@ class Sema final {
                         bool IsStdInitListInitialization, bool RequiresZeroInit,
                         unsigned ConstructKind, SourceRange ParenRange);
 
+  ExprResult ConvertMemberDefaultInitExpression(FieldDecl *FD, Expr *InitExpr,
+                                                SourceLocation InitLoc);
+
   ExprResult BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field);
 
 
   /// Instantiate or parse a C++ default argument expression as necessary.
   /// Return true on error.
   bool CheckCXXDefaultArgExpr(SourceLocation CallLoc, FunctionDecl *FD,
-                              ParmVarDecl *Param);
+                              ParmVarDecl *Param, Expr *Init = nullptr,
+                              bool SkipImmediateInvocations = true);
 
   /// BuildCXXDefaultArgExpr - Creates a CXXDefaultArgExpr, instantiating
   /// the default expr if needed.
-  ExprResult BuildCXXDefaultArgExpr(SourceLocation CallLoc,
-                                    FunctionDecl *FD,
-                                    ParmVarDecl *Param);
+  ExprResult BuildCXXDefaultArgExpr(SourceLocation CallLoc, FunctionDecl *FD,
+                                    ParmVarDecl *Param, Expr *Init = nullptr);
 
   /// FinalizeVarWithDestructor - Prepare for calling destructor on the
   /// constructed variable.
@@ -9627,6 +9649,63 @@ class Sema final {
     return ExprEvalContexts.back().isImmediateFunctionContext();
   }
 
+  bool isCheckingDefaultArgumentOrInitializer() const {
+    assert(!ExprEvalContexts.empty() &&
+           "Must be in an expression evaluation context");
+    const ExpressionEvaluationContextRecord &Ctx = ExprEvalContexts.back();
+    return (Ctx.Context ==
+            ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed) ||
+           Ctx.IsCurrentlyCheckingDefaultArgumentOrInitializer;
+  }
+
+  bool isCheckingDefaultArgumentOrInitializerOfOuterEntity() const {
+    assert(!ExprEvalContexts.empty() &&
+           "Must be in an expression evaluation context");
+    for (const auto &Ctx : llvm::reverse(ExprEvalContexts)) {
+      if ((Ctx.Context ==
+           ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed) ||
+          Ctx.IsCurrentlyCheckingDefaultArgumentOrInitializer)
+        return true;
+      if (Ctx.isConstantEvaluated() || Ctx.isImmediateFunctionContext() ||
+          Ctx.isUnevaluated())
+        return false;
+    }
+    return false;
+  }
+
+  llvm::Optional<ExpressionEvaluationContextRecord::InitializationContext>
+  InnermostDeclarationWithDelayedImmediateInvocations() const {
+    assert(!ExprEvalContexts.empty() &&
+           "Must be in an expression evaluation context");
+    for (const auto &Ctx : llvm::reverse(ExprEvalContexts)) {
+      if (Ctx.Context == ExpressionEvaluationContext::PotentiallyEvaluated &&
+          Ctx.DelayedDefaultInitializationContext)
+        return Ctx.DelayedDefaultInitializationContext;
+      if (Ctx.isConstantEvaluated() || Ctx.isImmediateFunctionContext() ||
+          Ctx.isUnevaluated())
+        break;
+    }
+    return std::nullopt;
+  }
+
+  llvm::Optional<ExpressionEvaluationContextRecord::InitializationContext>
+  OutermostDeclarationWithDelayedImmediateInvocations() const {
+    assert(!ExprEvalContexts.empty() &&
+           "Must be in an expression evaluation context");
+    llvm::Optional<ExpressionEvaluationContextRecord::InitializationContext>
+        Res;
+    for (auto &Ctx : llvm::reverse(ExprEvalContexts)) {
+      if (Ctx.Context == ExpressionEvaluationContext::PotentiallyEvaluated &&
+          !Ctx.DelayedDefaultInitializationContext && Res)
+        break;
+      if (Ctx.isConstantEvaluated() || Ctx.isImmediateFunctionContext() ||
+          Ctx.isUnevaluated())
+        break;
+      Res = Ctx.DelayedDefaultInitializationContext;
+    }
+    return Res;
+  }
+
   /// RAII class used to determine whether SFINAE has
   /// trapped any errors that occur during template argument
   /// deduction.

clang/lib/AST/ASTImporter.cpp

@@ -7687,9 +7687,16 @@ ExpectedStmt ASTNodeImporter::VisitCXXDefaultArgExpr(CXXDefaultArgExpr *E) {
     if (Error Err = ImportDefaultArgOfParmVarDecl(*FromParam, ToParam))
       return std::move(Err);
   }
-
+  Expr *RewrittenInit = nullptr;
+  if (E->hasRewrittenInit()) {
+    ExpectedExpr ExprOrErr = import(E->getExpr());
+    if (!ExprOrErr)
+      return ExprOrErr.takeError();
+    RewrittenInit = ExprOrErr.get();
+  }
   return CXXDefaultArgExpr::Create(Importer.getToContext(), *ToUsedLocOrErr,
-                                   *ToParamOrErr, *UsedContextOrErr);
+                                   *ToParamOrErr, RewrittenInit,
+                                   *UsedContextOrErr);
 }
 
 ExpectedStmt
@@ -8381,8 +8388,16 @@ ExpectedStmt ASTNodeImporter::VisitCXXDefaultInitExpr(CXXDefaultInitExpr *E) {
     ToField->setInClassInitializer(*ToInClassInitializerOrErr);
   }
 
+  Expr *RewrittenInit = nullptr;
+  if (E->hasRewrittenInit()) {
+    ExpectedExpr ExprOrErr = import(E->getExpr());
+    if (!ExprOrErr)
+      return ExprOrErr.takeError();
+    RewrittenInit = ExprOrErr.get();
+  }
+
   return CXXDefaultInitExpr::Create(Importer.getToContext(), *ToBeginLocOrErr,
-                                    ToField, *UsedContextOrErr);
+                                    ToField, *UsedContextOrErr, RewrittenInit);
 }
 
 ExpectedStmt ASTNodeImporter::VisitCXXNamedCastExpr(CXXNamedCastExpr *E) {

clang/lib/AST/Decl.cpp

@@ -2887,8 +2887,7 @@ Expr *ParmVarDecl::getDefaultArg() {
 
   Expr *Arg = getInit();
   if (auto *E = dyn_cast_or_null<FullExpr>(Arg))
-    if (!isa<ConstantExpr>(E))
-      return E->getSubExpr();
+    return E->getSubExpr();
 
   return Arg;
 }