[clang][Interp] Fix comparing one-past-the-end pointers

Author: tbaederr

clang/lib/AST/Interp/Pointer.h

@@ -513,6 +513,8 @@ class Pointer {
   unsigned getByteOffset() const {
     if (isIntegralPointer())
       return asIntPointer().Value + Offset;
+    if (isOnePastEnd())
+      return PastEndMark;
     return Offset;
   }
 

clang/test/AST/Interp/arrays.cpp

@@ -26,6 +26,9 @@ static_assert(foo[2][2] == nullptr, "");
 static_assert(foo[2][3] == &m, "");
 static_assert(foo[2][4] == nullptr, "");
 
+constexpr int afterEnd[] = {1,2,3};
+static_assert(&afterEnd[3] == afterEnd + 3, "");
+
 constexpr int ZeroSizeArray[] = {};
 
 constexpr int SomeInt[] = {1};