[clang][Interp] Check pointer inc/dec ops for null (#69168)

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1488,11 +1488,14 @@ static inline bool IncDecPtrHelper(InterpState &S, CodePtr OpPC,
                                    const Pointer &Ptr) {
   using OneT = Integral<8, false>;
 
+  const Pointer &P = Ptr.deref<Pointer>();
+  if (!CheckNull(S, OpPC, P, CSK_ArrayIndex))
+    return false;
+
   // Get the current value on the stack.
-  S.Stk.push<Pointer>(Ptr.deref<Pointer>());
+  S.Stk.push<Pointer>(P);
 
   // Now the current Ptr again and a constant 1.
-  Pointer P = Ptr.deref<Pointer>();
   OneT One = OneT::from(1);
   if (!OffsetHelper<OneT, Op>(S, OpPC, One, P))
     return false;

clang/test/AST/Interp/arrays.cpp

@@ -333,6 +333,26 @@ namespace IncDec {
                                    // expected-note {{in call to}} \
                                    // ref-error {{not an integral constant expression}} \
                                   // ref-note {{in call to}}
+
+  constexpr int nullptr1(bool Pre) {
+    int *a = nullptr;
+    if (Pre)
+      ++a; // ref-note {{arithmetic on null pointer}} \
+           // expected-note {{arithmetic on null pointer}}
+    else
+      a++; // ref-note {{arithmetic on null pointer}} \
+           // expected-note {{arithmetic on null pointer}}
+    return 1;
+  }
+  static_assert(nullptr1(true) == 1, ""); // ref-error {{not an integral constant expression}} \
+                                          // ref-note {{in call to}} \
+                                          // expected-error {{not an integral constant expression}} \
+                                          // expected-note {{in call to}}
+
+  static_assert(nullptr1(false) == 1, ""); // ref-error {{not an integral constant expression}} \
+                                           // ref-note {{in call to}} \
+                                           // expected-error {{not an integral constant expression}} \
+                                           // expected-note {{in call to}}
 };
 
 namespace ZeroInit {