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

[Shafik Yaghmour via cfe-commits]

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@@ -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) {
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shafik wrote:

Since we are now using a bit-field, initializing a bit-field using a default member initializer is a C++20 extension.

https://github.com/llvm/llvm-project/pull/95474