[clang][Interp] Fix nullptr array dereferencing (#75798)

The attached test case would cause an assertion failure in Pointer.h
when operating on a null pointer.

Author: tbaederr

clang/lib/AST/Interp/Interp.cpp

@@ -290,10 +290,10 @@ bool CheckInitialized(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
 }
 
 bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
-  if (!CheckDummy(S, OpPC, Ptr))
-    return false;
   if (!CheckLive(S, OpPC, Ptr, AK_Read))
     return false;
+  if (!CheckDummy(S, OpPC, Ptr))
+    return false;
   if (!CheckExtern(S, OpPC, Ptr))
     return false;
   if (!CheckRange(S, OpPC, Ptr, AK_Read))

clang/lib/AST/Interp/Interp.h

@@ -1813,9 +1813,6 @@ inline bool ArrayElemPtr(InterpState &S, CodePtr OpPC) {
   const T &Offset = S.Stk.pop<T>();
   const Pointer &Ptr = S.Stk.peek<Pointer>();
 
-  if (!CheckArray(S, OpPC, Ptr))
-    return false;
-
   if (!OffsetHelper<T, ArithOp::Add>(S, OpPC, Offset, Ptr))
     return false;
 
@@ -1843,9 +1840,6 @@ inline bool ArrayElemPtrPop(InterpState &S, CodePtr OpPC) {
   const T &Offset = S.Stk.pop<T>();
   const Pointer &Ptr = S.Stk.pop<Pointer>();
 
-  if (!CheckArray(S, OpPC, Ptr))
-    return false;
-
   if (!OffsetHelper<T, ArithOp::Add>(S, OpPC, Offset, Ptr))
     return false;
 

clang/test/AST/Interp/arrays.cpp

@@ -72,6 +72,14 @@ constexpr int getElementFromEnd(const int *Arr, int size, int index) {
 static_assert(getElementFromEnd(data, 5, 0) == 1, "");
 static_assert(getElementFromEnd(data, 5, 4) == 5, "");
 
+constexpr int getFirstElem(const int *a) {
+  return a[0]; // expected-note {{read of dereferenced null pointer}} \
+               // ref-note {{read of dereferenced null pointer}}
+}
+static_assert(getFirstElem(nullptr) == 1, ""); // expected-error {{not an integral constant expression}} \
+                                               // expected-note {{in call to}} \
+                                               // ref-error {{not an integral constant expression}} \
+                                               // ref-note {{in call to}}
 
 constexpr static int arr[2] = {1,2};
 constexpr static int arr2[2] = {3,4};