[clang][Interp] Handle one-past-the-end pointers in SubPtr

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1773,8 +1773,10 @@ inline bool SubPtr(InterpState &S, CodePtr OpPC) {
     return true;
   }
 
-  T A = T::from(LHS.getIndex());
-  T B = T::from(RHS.getIndex());
+  T A = LHS.isElementPastEnd() ? T::from(LHS.getNumElems())
+                               : T::from(LHS.getIndex());
+  T B = RHS.isElementPastEnd() ? T::from(RHS.getNumElems())
+                               : T::from(RHS.getIndex());
   return AddSubMulHelper<T, T::sub, std::minus>(S, OpPC, A.bitWidth(), A, B);
 }
 

clang/test/AST/Interp/arrays.cpp

@@ -609,3 +609,9 @@ namespace ArrayMemberAccess {
     bool cond = a->x;
   }
 }
+
+namespace OnePastEndSub {
+  struct A {};
+  constexpr A a[3][3];
+  constexpr int diff2 = &a[1][3] - &a[1][0]; /// Used to crash.
+}