[clang] Followup for constexpr-unknown potential constant expressions.

6a60f18 fixed the primary issue of
dereferences, but there are some expressions that depend on the
identity of the pointed-to object without actually accessing it. Handle
those cases.

Also, while I'm here, fix a crash in interpreter mode comparing typeid
to nullptr.

Author: efriedma-quic

clang/lib/AST/ByteCode/Pointer.cpp

@@ -563,8 +563,11 @@ bool Pointer::hasSameBase(const Pointer &A, const Pointer &B) {
   if (A.isTypeidPointer() && B.isTypeidPointer())
     return true;
 
-  if (A.isIntegralPointer() || B.isIntegralPointer())
+  if (A.isIntegralPointer() || B.isIntegralPointer()) {
+    if (A.isTypeidPointer() || B.isTypeidPointer())
+      return false;
     return A.getSource() == B.getSource();
+  }
 
   if (A.StorageKind != B.StorageKind)
     return false;

clang/lib/AST/ExprConstant.cpp

@@ -465,49 +465,7 @@ namespace {
     void diagnosePointerArithmetic(EvalInfo &Info, const Expr *E,
                                    const APSInt &N);
     /// Add N to the address of this subobject.
-    void adjustIndex(EvalInfo &Info, const Expr *E, APSInt N) {
-      if (Invalid || !N) return;
-      uint64_t TruncatedN = N.extOrTrunc(64).getZExtValue();
-      if (isMostDerivedAnUnsizedArray()) {
-        diagnoseUnsizedArrayPointerArithmetic(Info, E);
-        // Can't verify -- trust that the user is doing the right thing (or if
-        // not, trust that the caller will catch the bad behavior).
-        // FIXME: Should we reject if this overflows, at least?
-        Entries.back() = PathEntry::ArrayIndex(
-            Entries.back().getAsArrayIndex() + TruncatedN);
-        return;
-      }
-
-      // [expr.add]p4: For the purposes of these operators, a pointer to a
-      // nonarray object behaves the same as a pointer to the first element of
-      // an array of length one with the type of the object as its element type.
-      bool IsArray = MostDerivedPathLength == Entries.size() &&
-                     MostDerivedIsArrayElement;
-      uint64_t ArrayIndex = IsArray ? Entries.back().getAsArrayIndex()
-                                    : (uint64_t)IsOnePastTheEnd;
-      uint64_t ArraySize =
-          IsArray ? getMostDerivedArraySize() : (uint64_t)1;
-
-      if (N < -(int64_t)ArrayIndex || N > ArraySize - ArrayIndex) {
-        // Calculate the actual index in a wide enough type, so we can include
-        // it in the note.
-        N = N.extend(std::max<unsigned>(N.getBitWidth() + 1, 65));
-        (llvm::APInt&)N += ArrayIndex;
-        assert(N.ugt(ArraySize) && "bounds check failed for in-bounds index");
-        diagnosePointerArithmetic(Info, E, N);
-        setInvalid();
-        return;
-      }
-
-      ArrayIndex += TruncatedN;
-      assert(ArrayIndex <= ArraySize &&
-             "bounds check succeeded for out-of-bounds index");
-
-      if (IsArray)
-        Entries.back() = PathEntry::ArrayIndex(ArrayIndex);
-      else
-        IsOnePastTheEnd = (ArrayIndex != 0);
-    }
+    void adjustIndex(EvalInfo &Info, const Expr *E, APSInt N, LValue &LV);
   };
 
   /// A scope at the end of which an object can need to be destroyed.
@@ -1799,7 +1757,7 @@ namespace {
       Offset = CharUnits::fromQuantity(Offset64 + ElemSize64 * Index64);
 
       if (checkNullPointer(Info, E, CSK_ArrayIndex))
-        Designator.adjustIndex(Info, E, Index);
+        Designator.adjustIndex(Info, E, Index, *this);
       clearIsNullPointer();
     }
     void adjustOffset(CharUnits N) {
@@ -1907,6 +1865,54 @@ namespace {
   }
 }
 
+void SubobjectDesignator::adjustIndex(EvalInfo &Info, const Expr *E, APSInt N,
+                                      LValue &LV) {
+  if (Invalid || !N)
+    return;
+  uint64_t TruncatedN = N.extOrTrunc(64).getZExtValue();
+  if (isMostDerivedAnUnsizedArray()) {
+    diagnoseUnsizedArrayPointerArithmetic(Info, E);
+    // Can't verify -- trust that the user is doing the right thing (or if
+    // not, trust that the caller will catch the bad behavior).
+    // FIXME: Should we reject if this overflows, at least?
+    Entries.back() =
+        PathEntry::ArrayIndex(Entries.back().getAsArrayIndex() + TruncatedN);
+    return;
+  }
+
+  // [expr.add]p4: For the purposes of these operators, a pointer to a
+  // nonarray object behaves the same as a pointer to the first element of
+  // an array of length one with the type of the object as its element type.
+  bool IsArray =
+      MostDerivedPathLength == Entries.size() && MostDerivedIsArrayElement;
+  uint64_t ArrayIndex =
+      IsArray ? Entries.back().getAsArrayIndex() : (uint64_t)IsOnePastTheEnd;
+  uint64_t ArraySize = IsArray ? getMostDerivedArraySize() : (uint64_t)1;
+
+  if (N < -(int64_t)ArrayIndex || N > ArraySize - ArrayIndex) {
+    if (!Info.checkingPotentialConstantExpression() ||
+        !LV.AllowConstexprUnknown) {
+      // Calculate the actual index in a wide enough type, so we can include
+      // it in the note.
+      N = N.extend(std::max<unsigned>(N.getBitWidth() + 1, 65));
+      (llvm::APInt &)N += ArrayIndex;
+      assert(N.ugt(ArraySize) && "bounds check failed for in-bounds index");
+      diagnosePointerArithmetic(Info, E, N);
+    }
+    setInvalid();
+    return;
+  }
+
+  ArrayIndex += TruncatedN;
+  assert(ArrayIndex <= ArraySize &&
+         "bounds check succeeded for out-of-bounds index");
+
+  if (IsArray)
+    Entries.back() = PathEntry::ArrayIndex(ArrayIndex);
+  else
+    IsOnePastTheEnd = (ArrayIndex != 0);
+}
+
 static bool Evaluate(APValue &Result, EvalInfo &Info, const Expr *E);
 static bool EvaluateInPlace(APValue &Result, EvalInfo &Info,
                             const LValue &This, const Expr *E,
@@ -5126,12 +5132,18 @@ static bool HandleBaseToDerivedCast(EvalInfo &Info, const CastExpr *E,
   if (const PointerType *PT = TargetQT->getAs<PointerType>())
     TargetQT = PT->getPointeeType();
 
-  // Check this cast lands within the final derived-to-base subobject path.
-  if (D.MostDerivedPathLength + E->path_size() > D.Entries.size()) {
-    Info.CCEDiag(E, diag::note_constexpr_invalid_downcast)
-      << D.MostDerivedType << TargetQT;
+  auto InvalidCast = [&]() {
+    if (!Info.checkingPotentialConstantExpression() ||
+        !Result.AllowConstexprUnknown) {
+      Info.CCEDiag(E, diag::note_constexpr_invalid_downcast)
+          << D.MostDerivedType << TargetQT;
+    }
     return false;
-  }
+  };
+
+  // Check this cast lands within the final derived-to-base subobject path.
+  if (D.MostDerivedPathLength + E->path_size() > D.Entries.size())
+    return InvalidCast();
 
   // Check the type of the final cast. We don't need to check the path,
   // since a cast can only be formed if the path is unique.
@@ -5142,11 +5154,8 @@ static bool HandleBaseToDerivedCast(EvalInfo &Info, const CastExpr *E,
     FinalType = D.MostDerivedType->getAsCXXRecordDecl();
   else
     FinalType = getAsBaseClass(D.Entries[NewEntriesSize - 1]);
-  if (FinalType->getCanonicalDecl() != TargetType->getCanonicalDecl()) {
-    Info.CCEDiag(E, diag::note_constexpr_invalid_downcast)
-      << D.MostDerivedType << TargetQT;
-    return false;
-  }
+  if (FinalType->getCanonicalDecl() != TargetType->getCanonicalDecl())
+    return InvalidCast();
 
   // Truncate the lvalue to the appropriate derived class.
   return CastToDerivedClass(Info, E, Result, TargetType, NewEntriesSize);
@@ -6104,12 +6113,15 @@ static bool checkDynamicType(EvalInfo &Info, const Expr *E, const LValue &This,
     } else if (Polymorphic) {
       // Conservatively refuse to perform a polymorphic operation if we would
       // not be able to read a notional 'vptr' value.
-      APValue Val;
-      This.moveInto(Val);
-      QualType StarThisType =
-          Info.Ctx.getLValueReferenceType(This.Designator.getType(Info.Ctx));
-      Info.FFDiag(E, diag::note_constexpr_polymorphic_unknown_dynamic_type)
-          << AK << Val.getAsString(Info.Ctx, StarThisType);
+      if (!Info.checkingPotentialConstantExpression() ||
+          !This.AllowConstexprUnknown) {
+        APValue Val;
+        This.moveInto(Val);
+        QualType StarThisType =
+            Info.Ctx.getLValueReferenceType(This.Designator.getType(Info.Ctx));
+        Info.FFDiag(E, diag::note_constexpr_polymorphic_unknown_dynamic_type)
+            << AK << Val.getAsString(Info.Ctx, StarThisType);
+      }
       return false;
     }
     return true;
@@ -14554,6 +14566,11 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     // Reject differing bases from the normal codepath; we special-case
     // comparisons to null.
     if (!HasSameBase(LHSValue, RHSValue)) {
+      // Bail out early if we're checking potential constant expression.
+      // Otherwise, prefer to diagnose other issues.
+      if (Info.checkingPotentialConstantExpression() &&
+          (LHSValue.AllowConstexprUnknown || RHSValue.AllowConstexprUnknown))
+        return false;
       auto DiagComparison = [&] (unsigned DiagID, bool Reversed = false) {
         std::string LHS = LHSValue.toString(Info.Ctx, E->getLHS()->getType());
         std::string RHS = RHSValue.toString(Info.Ctx, E->getRHS()->getType());
@@ -14874,6 +14891,10 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
     // Reject differing bases from the normal codepath; we special-case
     // comparisons to null.
     if (!HasSameBase(LHSValue, RHSValue)) {
+      if (Info.checkingPotentialConstantExpression() &&
+          (LHSValue.AllowConstexprUnknown || RHSValue.AllowConstexprUnknown))
+        return false;
+
       const Expr *LHSExpr = LHSValue.Base.dyn_cast<const Expr *>();
       const Expr *RHSExpr = RHSValue.Base.dyn_cast<const Expr *>();
 

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++23 -verify=expected,nointerpreter %s
-// RUN: %clang_cc1 -std=c++23 -verify=expected,interpreter %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++23 -verify=expected,nointerpreter -Winvalid-constexpr %s
+// RUN: %clang_cc1 -std=c++23 -verify=expected,interpreter %s -fexperimental-new-constant-interpreter -Winvalid-constexpr
 
 using size_t = decltype(sizeof(0));
 
@@ -397,3 +397,37 @@ namespace GH150015 {
                                                  // nointerpreter-note {{comparison of addresses of subobjects of different base classes has unspecified value}} \
                                                  // interpreter-note {{initializer of 'z' is unknown}}
 }
+
+namespace InvalidConstexprFn {
+  // Make sure we don't trigger -Winvalid-constexpr incorrectly.
+  constexpr bool same_address(const int &a, const int &b) { return &a == &b; }
+  constexpr int next_element(const int &p) { return (&p)[2]; }
+
+  struct Base {};
+  struct Derived : Base { int n; };
+  constexpr int get_derived_member(const Base& b) { return static_cast<const Derived&>(b).n; }
+
+  struct PolyBase {
+    constexpr virtual int get() const { return 0; }
+  };
+  struct PolyDerived : PolyBase {
+    constexpr int get() const override { return 1; }
+  };
+  constexpr int virtual_call(const PolyBase& b) { return b.get(); }
+  constexpr auto* type(const PolyBase& b) { return &typeid(b); }
+  // FIXME: Intepreter doesn't support constexpr dynamic_cast yet.
+  constexpr const void* dyncast(const PolyBase& b) { return dynamic_cast<const void*>(&b); } // interpreter-error {{constexpr function never produces a constant expression}} \
+                                                                                             // interpreter-note 2 {{subexpression not valid in a constant expression}}
+  constexpr int sub(const int (&a)[], const int (&b)[]) { return a-b; }
+  constexpr const int* add(const int &a) { return &a+3; }
+
+  constexpr int arr[3]{0, 1, 2};
+  static_assert(same_address(arr[1], arr[1]));
+  static_assert(next_element(arr[0]) == 2);
+  static_assert(get_derived_member(Derived{}) == 0);
+  static_assert(virtual_call(PolyDerived{}) == 1);
+  static_assert(type(PolyDerived{}) != nullptr);
+  static_assert(dyncast(PolyDerived{}) != nullptr); // interpreter-error {{static assertion expression is not an integral constant expression}} interpreter-note {{in call}}
+  static_assert(sub(arr, arr) == 0);
+  static_assert(add(arr[0]) == &arr[3]);
+}