[clang][bytecode] Check GetPtrBase ops for null pointers (#110673)

Author: tbaederr

clang/lib/AST/ByteCode/Interp.h

@@ -1641,14 +1641,14 @@ inline bool GetPtrDerivedPop(InterpState &S, CodePtr OpPC, uint32_t Off) {
 
 inline bool GetPtrBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
   const Pointer &Ptr = S.Stk.peek<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
+    return false;
 
   if (!Ptr.isBlockPointer()) {
     S.Stk.push<Pointer>(Ptr.asIntPointer().baseCast(S.getASTContext(), Off));
     return true;
   }
 
-  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
-    return false;
   if (!CheckSubobject(S, OpPC, Ptr, CSK_Base))
     return false;
   const Pointer &Result = Ptr.atField(Off);
@@ -1661,13 +1661,14 @@ inline bool GetPtrBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
 inline bool GetPtrBasePop(InterpState &S, CodePtr OpPC, uint32_t Off) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
 
+  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
+    return false;
+
   if (!Ptr.isBlockPointer()) {
     S.Stk.push<Pointer>(Ptr.asIntPointer().baseCast(S.getASTContext(), Off));
     return true;
   }
 
-  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
-    return false;
   if (!CheckSubobject(S, OpPC, Ptr, CSK_Base))
     return false;
   const Pointer &Result = Ptr.atField(Off);

clang/test/AST/ByteCode/records.cpp

@@ -1653,3 +1653,11 @@ namespace ExprWithCleanups {
   constexpr auto F = true ? 1i : 2i;
   static_assert(F == 1i, "");
 }
+
+namespace NullptrUpcast {
+  struct A {};
+  struct B : A { int n; };
+  constexpr B *nb = nullptr;
+  constexpr A &ra = *nb; // both-error {{constant expression}} \
+                         // both-note {{cannot access base class of null pointer}}
+}