[clang][Interp] Diagnose comparisons against one-past-end pointers

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -922,6 +922,7 @@ inline bool CmpHelperEQ<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
     return true;
   }
 
+  // Reject comparisons to weak pointers.
   for (const auto &P : {LHS, RHS}) {
     if (P.isZero())
       continue;
@@ -934,6 +935,20 @@ inline bool CmpHelperEQ<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
   }
 
   if (!Pointer::hasSameBase(LHS, RHS)) {
+    if (LHS.isOnePastEnd() && !RHS.isOnePastEnd() && !RHS.isZero() &&
+        RHS.getOffset() == 0) {
+      const SourceInfo &Loc = S.Current->getSource(OpPC);
+      S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_past_end)
+          << LHS.toDiagnosticString(S.getCtx());
+      return false;
+    } else if (RHS.isOnePastEnd() && !LHS.isOnePastEnd() && !LHS.isZero() &&
+               LHS.getOffset() == 0) {
+      const SourceInfo &Loc = S.Current->getSource(OpPC);
+      S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_past_end)
+          << RHS.toDiagnosticString(S.getCtx());
+      return false;
+    }
+
     S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Unordered)));
     return true;
   } else {

clang/lib/AST/Interp/Pointer.cpp

@@ -143,7 +143,7 @@ APValue Pointer::toAPValue() const {
 
   if (isDummy() || isUnknownSizeArray() || Desc->asExpr())
     return APValue(Base, CharUnits::Zero(), Path,
-                   /*IsOnePastEnd=*/false, /*IsNullPtr=*/false);
+                   /*IsOnePastEnd=*/isOnePastEnd(), /*IsNullPtr=*/false);
 
   // TODO: compute the offset into the object.
   CharUnits Offset = CharUnits::Zero();
@@ -181,7 +181,8 @@ APValue Pointer::toAPValue() const {
   // Just invert the order of the elements.
   std::reverse(Path.begin(), Path.end());
 
-  return APValue(Base, Offset, Path, /*IsOnePastEnd=*/false, /*IsNullPtr=*/false);
+  return APValue(Base, Offset, Path, /*IsOnePastEnd=*/isOnePastEnd(),
+                 /*IsNullPtr=*/false);
 }
 
 void Pointer::print(llvm::raw_ostream &OS) const {
@@ -348,7 +349,7 @@ std::optional<APValue> Pointer::toRValue(const Context &Ctx) const {
 
     // Invalid pointers.
     if (Ptr.isDummy() || !Ptr.isLive() || !Ptr.isBlockPointer() ||
-        (!Ptr.isUnknownSizeArray() && Ptr.isOnePastEnd()))
+        Ptr.isPastEnd())
       return false;
 
     // Primitive values.

clang/lib/AST/Interp/Pointer.h

@@ -556,10 +556,18 @@ class Pointer {
     if (isUnknownSizeArray())
       return false;
 
-    return isElementPastEnd() ||
+    return isElementPastEnd() || isPastEnd() ||
            (getSize() == getOffset() && !isZeroSizeArray());
   }
 
+  /// Checks if the pointer points past the end of the object.
+  bool isPastEnd() const {
+    if (isIntegralPointer())
+      return false;
+
+    return !isZero() && Offset > PointeeStorage.BS.Pointee->getSize();
+  }
+
   /// Checks if the pointer is an out-of-bounds element pointer.
   bool isElementPastEnd() const { return Offset == PastEndMark; }
 

clang/test/AST/Interp/literals.cpp

@@ -1266,3 +1266,13 @@ static_assert(ReturnInStmtExpr() == 1, ""); // both-error {{not an integral cons
                                             // both-note {{in call to}}
 
 #endif
+
+namespace ComparisonAgainstOnePastEnd {
+  int a, b;
+  static_assert(&a + 1 == &b, ""); // both-error {{not an integral constant expression}} \
+                                   // both-note {{comparison against pointer '&a + 1' that points past the end of a complete object has unspecified value}}
+  static_assert(&a == &b + 1, ""); // both-error {{not an integral constant expression}} \
+                                   // both-note {{comparison against pointer '&b + 1' that points past the end of a complete object has unspecified value}}
+
+  static_assert(&a + 1 == &b + 1, ""); // both-error {{static assertion failed}}
+};