[Clang] Implement P2280R4 Using unknown pointers and references in constant expressions

P2280R4 allows the use of references in pointers of unknown origins in a
constant expression context but only in specific cases that could be constant
expressions.

We track whether a variable is a constexpr unknown in a constant expression by
setting a flag in either APValue or LValue and using this flag to prevent using
unknown values in places where it is not allowed.

In `evaluateVarDeclInit` we may need to create a new `APValue` to track the
unknown referene or pointer and we track that `APValue` in the
`CallStackFrame`.

Fixes: #63139
#63117

Author: shafik

clang/include/clang/AST/APValue.h

@@ -249,13 +249,15 @@ class APValue {
   struct NoLValuePath {};
   struct UninitArray {};
   struct UninitStruct {};
+  struct ConstexprUnknown {};
 
   template <typename Impl> friend class clang::serialization::BasicReaderBase;
   friend class ASTImporter;
   friend class ASTNodeImporter;
 
 private:
   ValueKind Kind;
+  bool AllowConstexprUnknown = false;
 
   struct ComplexAPSInt {
     APSInt Real, Imag;
@@ -314,53 +316,69 @@ class APValue {
   DataType Data;
 
 public:
-  APValue() : Kind(None) {}
-  explicit APValue(APSInt I) : Kind(None) {
+  bool allowConstexprUnknown() const { return AllowConstexprUnknown; }
+
+  void setConstexprUnknown() { AllowConstexprUnknown = true; }
+
+  APValue() : Kind(None), AllowConstexprUnknown(false) {}
+  explicit APValue(APSInt I) : Kind(None), AllowConstexprUnknown(false) {
     MakeInt(); setInt(std::move(I));
   }
-  explicit APValue(APFloat F) : Kind(None) {
+  explicit APValue(APFloat F) : Kind(None), AllowConstexprUnknown(false) {
     MakeFloat(); setFloat(std::move(F));
   }
-  explicit APValue(APFixedPoint FX) : Kind(None) {
+  explicit APValue(APFixedPoint FX) : Kind(None), AllowConstexprUnknown(false) {
     MakeFixedPoint(std::move(FX));
   }
-  explicit APValue(const APValue *E, unsigned N) : Kind(None) {
+  explicit APValue(const APValue *E, unsigned N)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeVector(); setVector(E, N);
   }
-  APValue(APSInt R, APSInt I) : Kind(None) {
+  APValue(APSInt R, APSInt I) : Kind(None), AllowConstexprUnknown(false) {
     MakeComplexInt(); setComplexInt(std::move(R), std::move(I));
   }
-  APValue(APFloat R, APFloat I) : Kind(None) {
+  APValue(APFloat R, APFloat I) : Kind(None), AllowConstexprUnknown(false) {
     MakeComplexFloat(); setComplexFloat(std::move(R), std::move(I));
   }
   APValue(const APValue &RHS);
   APValue(APValue &&RHS);
   APValue(LValueBase B, const CharUnits &O, NoLValuePath N,
           bool IsNullPtr = false)
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeLValue(); setLValue(B, O, N, IsNullPtr);
   }
   APValue(LValueBase B, const CharUnits &O, ArrayRef<LValuePathEntry> Path,
           bool OnePastTheEnd, bool IsNullPtr = false)
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeLValue(); setLValue(B, O, Path, OnePastTheEnd, IsNullPtr);
   }
-  APValue(UninitArray, unsigned InitElts, unsigned Size) : Kind(None) {
+
+  APValue(LValueBase B, ConstexprUnknown, const CharUnits &O,
+          bool IsNullPtr = false)
+      : Kind(None), AllowConstexprUnknown(true) {
+    MakeLValue();
+    setLValue(B, O, NoLValuePath{}, IsNullPtr);
+  }
+
+  APValue(UninitArray, unsigned InitElts, unsigned Size)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeArray(InitElts, Size);
   }
-  APValue(UninitStruct, unsigned B, unsigned M) : Kind(None) {
+  APValue(UninitStruct, unsigned B, unsigned M)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeStruct(B, M);
   }
   explicit APValue(const FieldDecl *D, const APValue &V = APValue())
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeUnion(); setUnion(D, V);
   }
   APValue(const ValueDecl *Member, bool IsDerivedMember,
-          ArrayRef<const CXXRecordDecl*> Path) : Kind(None) {
+          ArrayRef<const CXXRecordDecl *> Path)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeMemberPointer(Member, IsDerivedMember, Path);
   }
-  APValue(const AddrLabelExpr* LHSExpr, const AddrLabelExpr* RHSExpr)
-      : Kind(None) {
+  APValue(const AddrLabelExpr *LHSExpr, const AddrLabelExpr *RHSExpr)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeAddrLabelDiff(); setAddrLabelDiff(LHSExpr, RHSExpr);
   }
   static APValue IndeterminateValue() {

clang/lib/AST/APValue.cpp

@@ -308,7 +308,8 @@ APValue::UnionData::~UnionData () {
   delete Value;
 }
 
-APValue::APValue(const APValue &RHS) : Kind(None) {
+APValue::APValue(const APValue &RHS)
+    : Kind(None), AllowConstexprUnknown(RHS.AllowConstexprUnknown) {
   switch (RHS.getKind()) {
   case None:
   case Indeterminate:
@@ -379,13 +380,17 @@ APValue::APValue(const APValue &RHS) : Kind(None) {
   }
 }
 
-APValue::APValue(APValue &&RHS) : Kind(RHS.Kind), Data(RHS.Data) {
+APValue::APValue(APValue &&RHS)
+    : Kind(RHS.Kind), AllowConstexprUnknown(RHS.AllowConstexprUnknown),
+      Data(RHS.Data) {
   RHS.Kind = None;
 }
 
 APValue &APValue::operator=(const APValue &RHS) {
   if (this != &RHS)
     *this = APValue(RHS);
+
+  AllowConstexprUnknown = RHS.AllowConstexprUnknown;
   return *this;
 }
 
@@ -395,6 +400,7 @@ APValue &APValue::operator=(APValue &&RHS) {
       DestroyDataAndMakeUninit();
     Kind = RHS.Kind;
     Data = RHS.Data;
+    AllowConstexprUnknown = RHS.AllowConstexprUnknown;
     RHS.Kind = None;
   }
   return *this;
@@ -426,6 +432,7 @@ void APValue::DestroyDataAndMakeUninit() {
   else if (Kind == AddrLabelDiff)
     ((AddrLabelDiffData *)(char *)&Data)->~AddrLabelDiffData();
   Kind = None;
+  AllowConstexprUnknown = false;
 }
 
 bool APValue::needsCleanup() const {
@@ -468,6 +475,7 @@ bool APValue::needsCleanup() const {
 void APValue::swap(APValue &RHS) {
   std::swap(Kind, RHS.Kind);
   std::swap(Data, RHS.Data);
+  std::swap(AllowConstexprUnknown, RHS.AllowConstexprUnknown);
 }
 
 /// Profile the value of an APInt, excluding its bit-width.

clang/lib/AST/ExprConstant.cpp

@@ -1609,8 +1609,11 @@ namespace {
     SubobjectDesignator Designator;
     bool IsNullPtr : 1;
     bool InvalidBase : 1;
+    // P2280R4 track if we have an unknown reference or pointer.
+    bool AllowConstexprUnknown = false;
 
     const APValue::LValueBase getLValueBase() const { return Base; }
+    bool allowConstexprUnknown() const { return AllowConstexprUnknown; }
     CharUnits &getLValueOffset() { return Offset; }
     const CharUnits &getLValueOffset() const { return Offset; }
     SubobjectDesignator &getLValueDesignator() { return Designator; }
@@ -1628,6 +1631,8 @@ namespace {
         V = APValue(Base, Offset, Designator.Entries,
                     Designator.IsOnePastTheEnd, IsNullPtr);
       }
+      if (AllowConstexprUnknown)
+        V.setConstexprUnknown();
     }
     void setFrom(ASTContext &Ctx, const APValue &V) {
       assert(V.isLValue() && "Setting LValue from a non-LValue?");
@@ -1636,6 +1641,7 @@ namespace {
       InvalidBase = false;
       Designator = SubobjectDesignator(Ctx, V);
       IsNullPtr = V.isNullPointer();
+      AllowConstexprUnknown = V.allowConstexprUnknown();
     }
 
     void set(APValue::LValueBase B, bool BInvalid = false) {
@@ -1653,6 +1659,7 @@ namespace {
       InvalidBase = BInvalid;
       Designator = SubobjectDesignator(getType(B));
       IsNullPtr = false;
+      AllowConstexprUnknown = false;
     }
 
     void setNull(ASTContext &Ctx, QualType PointerTy) {
@@ -1662,6 +1669,7 @@ namespace {
       InvalidBase = false;
       Designator = SubobjectDesignator(PointerTy->getPointeeType());
       IsNullPtr = true;
+      AllowConstexprUnknown = false;
     }
 
     void setInvalid(APValue::LValueBase B, unsigned I = 0) {
@@ -3300,6 +3308,11 @@ static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
 static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
                                 const VarDecl *VD, CallStackFrame *Frame,
                                 unsigned Version, APValue *&Result) {
+  // P2280R4 If we have a reference type and we are in C++23 allow unknown
+  // references and pointers.
+  bool AllowConstexprUnknown =
+      Info.getLangOpts().CPlusPlus23 && VD->getType()->isReferenceType();
+
   APValue::LValueBase Base(VD, Frame ? Frame->Index : 0, Version);
 
   // If this is a local variable, dig out its value.
@@ -3334,7 +3347,9 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
     return true;
   }
 
-  if (isa<ParmVarDecl>(VD)) {
+  // P2280R4 struck the restriction that variable of referene type lifetime
+  // should begin within the evaluation of E
+  if (isa<ParmVarDecl>(VD) && !AllowConstexprUnknown) {
     // Assume parameters of a potential constant expression are usable in
     // constant expressions.
     if (!Info.checkingPotentialConstantExpression() ||
@@ -3358,7 +3373,9 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   // FIXME: We should eventually check whether the variable has a reachable
   // initializing declaration.
   const Expr *Init = VD->getAnyInitializer(VD);
-  if (!Init) {
+  // P2280R4 struck the restriction that variable of referene type should have
+  // a preceding initialization.
+  if (!Init && !AllowConstexprUnknown) {
     // Don't diagnose during potential constant expression checking; an
     // initializer might be added later.
     if (!Info.checkingPotentialConstantExpression()) {
@@ -3369,7 +3386,9 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
     return false;
   }
 
-  if (Init->isValueDependent()) {
+  // P2280R4 struck the initialization requirement for variables of reference
+  // type so we can no longer assume we have an Init.
+  if (Init && Init->isValueDependent()) {
     // The DeclRefExpr is not value-dependent, but the variable it refers to
     // has a value-dependent initializer. This should only happen in
     // constant-folding cases, where the variable is not actually of a suitable
@@ -3388,7 +3407,9 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
 
   // Check that we can fold the initializer. In C++, we will have already done
   // this in the cases where it matters for conformance.
-  if (!VD->evaluateValue()) {
+  // P2280R4 struck the initialization requirement for variables of reference
+  // type so we can no longer assume we have an Init.
+  if (Init && !VD->evaluateValue()) {
     Info.FFDiag(E, diag::note_constexpr_var_init_non_constant, 1) << VD;
     NoteLValueLocation(Info, Base);
     return false;
@@ -3420,6 +3441,15 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   }
 
   Result = VD->getEvaluatedValue();
+
+  // P2280R4 If we don't have a value because this is a reference that was not
+  // initialized or whose lifetime began within E then create a value with as
+  // a ConstexprUnknown status.
+  if (AllowConstexprUnknown) {
+    if (!Result) {
+      Result = new APValue(Base, APValue::ConstexprUnknown{}, CharUnits::One());
+    }
+  }
   return true;
 }
 
@@ -3700,6 +3730,11 @@ findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
   const FieldDecl *LastField = nullptr;
   const FieldDecl *VolatileField = nullptr;
 
+  // P2280R4 If we have an unknown referene or pointer and we don't have a
+  // value then bail out.
+  if (O->allowConstexprUnknown() && !O->hasValue())
+    return false;
+
   // Walk the designator's path to find the subobject.
   for (unsigned I = 0, N = Sub.Entries.size(); /**/; ++I) {
     // Reading an indeterminate value is undefined, but assigning over one is OK.
@@ -5732,6 +5767,12 @@ struct CheckDynamicTypeHandler {
 /// dynamic type.
 static bool checkDynamicType(EvalInfo &Info, const Expr *E, const LValue &This,
                              AccessKinds AK, bool Polymorphic) {
+  // P2280R4 We are not allowed to invoke a virtual function whose dynamic type
+  // us constexpr-unknown, so stop early and let this fail later on if we
+  // attempt to do so.
+  if (This.allowConstexprUnknown())
+    return true;
+
   if (This.Designator.Invalid)
     return false;
 
@@ -5804,7 +5845,13 @@ static std::optional<DynamicType> ComputeDynamicType(EvalInfo &Info,
   // If we don't have an lvalue denoting an object of class type, there is no
   // meaningful dynamic type. (We consider objects of non-class type to have no
   // dynamic type.)
-  if (!checkDynamicType(Info, E, This, AK, true))
+  if (!checkDynamicType(Info, E, This, AK,
+                        (AK == AK_TypeId
+                             ? (E->getType()->isReferenceType() ? true : false)
+                             : true)))
+    return std::nullopt;
+
+  if (This.Designator.Invalid)
     return std::nullopt;
 
   // Refuse to compute a dynamic type in the presence of virtual bases. This
@@ -8539,7 +8586,8 @@ static bool HandleLambdaCapture(EvalInfo &Info, const Expr *E, LValue &Result,
     const ParmVarDecl *Self = MD->getParamDecl(0);
     if (Self->getType()->isReferenceType()) {
       APValue *RefValue = Info.getParamSlot(Info.CurrentCall->Arguments, Self);
-      Result.setFrom(Info.Ctx, *RefValue);
+      if (!RefValue->allowConstexprUnknown() || RefValue->hasValue())
+        Result.setFrom(Info.Ctx, *RefValue);
     } else {
       const ParmVarDecl *VD = Info.CurrentCall->Arguments.getOrigParam(Self);
       CallStackFrame *Frame =
@@ -8595,7 +8643,10 @@ bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
 
 
 bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
-
+  // P2280R4 if we are in C++23 track if we have an unknown reference or
+  // pointer.
+  bool AllowConstexprUnknown =
+      Info.getLangOpts().CPlusPlus23 && VD->getType()->isReferenceType();
   // If we are within a lambda's call operator, check whether the 'VD' referred
   // to within 'E' actually represents a lambda-capture that maps to a
   // data-member/field within the closure object, and if so, evaluate to the
@@ -8665,10 +8716,23 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
   if (!V->hasValue()) {
     // FIXME: Is it possible for V to be indeterminate here? If so, we should
     // adjust the diagnostic to say that.
-    if (!Info.checkingPotentialConstantExpression())
+    // P2280R4 If we are have a variable that is unknown reference or pointer
+    // it may not have a value but still be usable later on so do not diagnose.
+    if (!Info.checkingPotentialConstantExpression() && !AllowConstexprUnknown)
       Info.FFDiag(E, diag::note_constexpr_use_uninit_reference);
+
+    // P2280R4 If we are have a variable that is unknown reference or pointer
+    // try to recover it from the frame and set the result accordingly.
+    if (VD->getType()->isReferenceType() && AllowConstexprUnknown) {
+      if (Frame) {
+        Result.set({VD, Frame->Index, Version});
+        return true;
+      }
+      return Success(VD);
+    }
     return false;
   }
+
   return Success(*V, E);
 }
 
@@ -11486,7 +11550,10 @@ class IntExprEvaluator
   }
 
   bool Success(const APValue &V, const Expr *E) {
-    if (V.isLValue() || V.isAddrLabelDiff() || V.isIndeterminate()) {
+    // P2280R4 if we have an unknown reference or pointer allow further
+    // evaluation of the value.
+    if (V.isLValue() || V.isAddrLabelDiff() || V.isIndeterminate() ||
+        V.allowConstexprUnknown()) {
       Result = V;
       return true;
     }