[Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (#95474)

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.

Fixes: #63139 #63117

Author: shafik

clang/docs/ReleaseNotes.rst

@@ -318,6 +318,8 @@ C++23 Feature Support
 
 - ``__cpp_explicit_this_parameter`` is now defined. (#GH82780)
 
+- Add support for `P2280R4 Using unknown pointers and references in constant expressions <https://wg21.link/P2280R4>`_. (#GH63139)
+
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/APValue.h

@@ -247,13 +247,15 @@ 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 : 1;
 
   struct ComplexAPSInt {
     APSInt Real, Imag;
@@ -312,32 +314,39 @@ class APValue {
   DataType Data;
 
 public:
+  bool allowConstexprUnknown() const { return AllowConstexprUnknown; }
+
+  void setConstexprUnknown(bool IsConstexprUnknown = true) {
+    AllowConstexprUnknown = IsConstexprUnknown;
+  }
+
   /// Creates an empty APValue of type None.
-  APValue() : Kind(None) {}
+  APValue() : Kind(None), AllowConstexprUnknown(false) {}
   /// Creates an integer APValue holding the given value.
-  explicit APValue(APSInt I) : Kind(None) {
+  explicit APValue(APSInt I) : Kind(None), AllowConstexprUnknown(false) {
     MakeInt(); setInt(std::move(I));
   }
   /// Creates a float APValue holding the given value.
-  explicit APValue(APFloat F) : Kind(None) {
+  explicit APValue(APFloat F) : Kind(None), AllowConstexprUnknown(false) {
     MakeFloat(); setFloat(std::move(F));
   }
   /// Creates a fixed-point APValue holding the given value.
-  explicit APValue(APFixedPoint FX) : Kind(None) {
+  explicit APValue(APFixedPoint FX) : Kind(None), AllowConstexprUnknown(false) {
     MakeFixedPoint(std::move(FX));
   }
   /// Creates a vector APValue with \p N elements. The elements
   /// are read from \p E.
-  explicit APValue(const APValue *E, unsigned N) : Kind(None) {
+  explicit APValue(const APValue *E, unsigned N)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeVector(); setVector(E, N);
   }
   /// Creates an integer complex APValue with the given real and imaginary
   /// values.
-  APValue(APSInt R, APSInt I) : Kind(None) {
+  APValue(APSInt R, APSInt I) : Kind(None), AllowConstexprUnknown(false) {
     MakeComplexInt(); setComplexInt(std::move(R), std::move(I));
   }
   /// Creates a float complex APValue with the given real and imaginary values.
-  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);
@@ -348,7 +357,7 @@ class APValue {
   /// \param IsNullPtr Whether this lvalue is a null pointer.
   APValue(LValueBase Base, const CharUnits &Offset, NoLValuePath,
           bool IsNullPtr = false)
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeLValue();
     setLValue(Base, Offset, NoLValuePath{}, IsNullPtr);
   }
@@ -362,31 +371,44 @@ class APValue {
   APValue(LValueBase Base, const CharUnits &Offset,
           ArrayRef<LValuePathEntry> Path, bool OnePastTheEnd,
           bool IsNullPtr = false)
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeLValue();
     setLValue(Base, Offset, Path, OnePastTheEnd, IsNullPtr);
   }
+  /// Creates a constexpr unknown lvalue APValue.
+  /// \param Base The base of the lvalue.
+  /// \param Offset The offset of the lvalue.
+  /// \param IsNullPtr Whether this lvalue is a null pointer.
+  APValue(LValueBase Base, const CharUnits &Offset, ConstexprUnknown,
+          bool IsNullPtr = false)
+      : Kind(None), AllowConstexprUnknown(true) {
+    MakeLValue();
+    setLValue(Base, Offset, NoLValuePath{}, IsNullPtr);
+  }
+
   /// Creates a new array APValue.
   /// \param UninitArray Marker. Pass an empty UninitArray.
   /// \param InitElts Number of elements you're going to initialize in the
   /// array.
   /// \param Size Full size of the array.
-  APValue(UninitArray, unsigned InitElts, unsigned Size) : Kind(None) {
+  APValue(UninitArray, unsigned InitElts, unsigned Size)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeArray(InitElts, Size);
   }
   /// Creates a new struct APValue.
   /// \param UninitStruct Marker. Pass an empty UninitStruct.
   /// \param NumBases Number of bases.
   /// \param NumMembers Number of members.
-  APValue(UninitStruct, unsigned NumBases, unsigned NumMembers) : Kind(None) {
+  APValue(UninitStruct, unsigned NumBases, unsigned NumMembers)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeStruct(NumBases, NumMembers);
   }
   /// Creates a new union APValue.
   /// \param ActiveDecl The FieldDecl of the active union member.
   /// \param ActiveValue The value of the active union member.
   explicit APValue(const FieldDecl *ActiveDecl,
                    const APValue &ActiveValue = APValue())
-      : Kind(None) {
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeUnion();
     setUnion(ActiveDecl, ActiveValue);
   }
@@ -395,14 +417,15 @@ class APValue {
   /// \param IsDerivedMember Whether member is a derived one.
   /// \param Path The path of the member.
   APValue(const ValueDecl *Member, bool IsDerivedMember,
-          ArrayRef<const CXXRecordDecl*> Path) : Kind(None) {
+          ArrayRef<const CXXRecordDecl *> Path)
+      : Kind(None), AllowConstexprUnknown(false) {
     MakeMemberPointer(Member, IsDerivedMember, Path);
   }
   /// Creates a new address label diff APValue.
   /// \param LHSExpr The left-hand side of the difference.
   /// \param RHSExpr The right-hand side of the difference.
-  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 +475,10 @@ bool APValue::needsCleanup() const {
 void APValue::swap(APValue &RHS) {
   std::swap(Kind, RHS.Kind);
   std::swap(Data, RHS.Data);
+  // We can't use std::swap w/ bit-fields
+  bool tmp = AllowConstexprUnknown;
+  AllowConstexprUnknown = RHS.AllowConstexprUnknown;
+  RHS.AllowConstexprUnknown = tmp;
 }
 
 /// Profile the value of an APInt, excluding its bit-width.