[clang][Interp] Emit proper diagnostic when comparing unrelated pointers

[tbaeder]

Summary

Also adds toDiagnosticString() to Pointer so we can use that when emitting the diagnostic.

[aaron.ballman]

LGTM!

[tbaeder]

Meh, turns out (void*)&s.a; is a bitcast, so can't push this yet.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
   case CK_ArrayToPointerDecay:
   case CK_AtomicToNonAtomic:
   case CK_ConstructorConversion:
   case CK_FunctionToPointerDecay:
   case CK_NonAtomicToAtomic:
   case CK_NoOp:
   case CK_UserDefinedConversion:
+  case CK_BitCast:
     return this->visit(SubExpr);
 
   case CK_IntegralToBoolean:
   case CK_IntegralCast: {
     std::optional<PrimType> FromT = classify(SubExpr->getType());
     std::optional<PrimType> ToT = classify(CE->getType());
     if (!FromT || !ToT)
       return false;

clang/lib/AST/Interp/Interp.h

 template <>
 inline bool CmpHelper<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
   using BoolT = PrimConv<PT_Bool>::T;
   const Pointer &RHS = S.Stk.pop<Pointer>();
   const Pointer &LHS = S.Stk.pop<Pointer>();
 
   if (!Pointer::hasSameBase(LHS, RHS)) {
     const SourceInfo &Loc = S.Current->getSource(OpPC);
-    S.FFDiag(Loc, diag::note_invalid_subexpr_in_const_expr);
+    S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_unspecified)
+        << LHS.toDiagnosticString(S.getCtx())
+        << RHS.toDiagnosticString(S.getCtx());
     return false;
   } else {
     unsigned VL = LHS.getByteOffset();
     unsigned VR = RHS.getByteOffset();
     S.Stk.push<BoolT>(BoolT::from(Fn(Compare(VL, VR))));
     return true;
   }
 }

clang/lib/AST/Interp/Pointer.cpp

APValue Pointer::toAPValue() const {
   // outermost one. SO in a.b.c, the first element in Path will refer to
   // the field 'c', while later code expects it to refer to 'a'.
   // Just invert the order of the elements.
   std::reverse(Path.begin(), Path.end());
 
   return APValue(Base, Offset, Path, IsOnePastEnd, IsNullPtr);
 }
 
+std::string Pointer::toDiagnosticString(const ASTContext &Ctx) const {
+  if (!Pointee)
+    return "nullptr";
+
+  return toAPValue().getAsString(Ctx, getType());
+}
+
 bool Pointer::isInitialized() const {
   assert(Pointee && "Cannot check if null pointer was initialized");
   const Descriptor *Desc = getFieldDesc();
   assert(Desc);
   if (Desc->isPrimitiveArray()) {
     if (isStatic() && Base == 0)
       return true;
     // Primitive array field are stored in a bitset.

clang/test/AST/Interp/literals.cpp

namespace PointerToBool {
   static_assert(!N, "");
 
   constexpr float F = 1.0;
   constexpr const float *FP = &F;
   static_assert(FP, "");
   static_assert(!!FP, "");
 }
 
+namespace PointerComparison {
+
+  struct S { int a, b; } s;
+  constexpr void *null = 0;
+  constexpr void *pv = (void*)&s.a;
+  constexpr void *qv = (void*)&s.b;
+  constexpr bool v1 = null < (int*)0;
+  constexpr bool v2 = null < pv; // expected-error {{must be initialized by a constant expression}} \
+                                 // expected-note {{comparison between 'nullptr' and '&s.a' has unspecified value}} \
+                                 // ref-error {{must be initialized by a constant expression}} \
+                                 // ref-note {{comparison between 'nullptr' and '&s.a' has unspecified value}} \
+
+  constexpr bool v3 = null == pv; // ok
+  constexpr bool v4 = qv == pv; // ok
+
+  /// FIXME: These two are rejected by the current interpreter, but
+  ///   accepted by GCC.
+  constexpr bool v5 = qv >= pv; // ref-error {{constant expression}} \
+                                // ref-note {{unequal pointers to void}}
+  constexpr bool v8 = qv > (void*)&s.a; // ref-error {{constant expression}} \
+                                        // ref-note {{unequal pointers to void}}
+  constexpr bool v6 = qv > null; // expected-error {{must be initialized by a constant expression}} \
+                                 // expected-note {{comparison between '&s.b' and 'nullptr' has unspecified value}} \
+                                 // ref-error {{must be initialized by a constant expression}} \
+                                 // ref-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
+
+  constexpr bool v7 = qv <= (void*)&s.b; // ok
+}
+
 namespace SizeOf {
   constexpr int soint = sizeof(int);
   constexpr int souint = sizeof(unsigned int);
   static_assert(soint == souint, "");
 
   static_assert(sizeof(&soint) == sizeof(void*), "");
   static_assert(sizeof(&soint) == sizeof(nullptr), "");