[clang][bytecode] Handle reads on zero-size arrays (#152706)

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -518,7 +518,7 @@ bool CheckNull(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
 
 bool CheckRange(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
                 AccessKinds AK) {
-  if (!Ptr.isOnePastEnd())
+  if (!Ptr.isOnePastEnd() && !Ptr.isZeroSizeArray())
     return true;
   if (S.getLangOpts().CPlusPlus) {
     const SourceInfo &Loc = S.Current->getSource(OpPC);
@@ -829,8 +829,6 @@ bool CheckFinalLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
     return false;
   if (!CheckExtern(S, OpPC, Ptr))
     return false;
-  if (!CheckRange(S, OpPC, Ptr, AK_Read))
-    return false;
   if (!CheckActive(S, OpPC, Ptr, AK_Read))
     return false;
   if (!CheckLifetime(S, OpPC, Ptr.getLifetime(), AK_Read))

clang/test/AST/ByteCode/arrays.cpp

@@ -779,3 +779,20 @@ namespace DiscardedSubScriptExpr {
     return true;
   }
 }
+
+namespace ZeroSizeArrayRead {
+  constexpr char str[0] = {};
+  constexpr  unsigned checksum(const char *s) {
+    unsigned result = 0;
+    for (const char *p = s; *p != '\0'; ++p) { // both-note {{read of dereferenced one-past-the-end pointer}}
+      result += *p;
+    }
+    return result;
+  }
+  constexpr unsigned C = checksum(str); // both-error {{must be initialized by a constant expression}} \
+                                        // both-note {{in call to}}
+
+  constexpr const char *p1 = &str[0];
+  constexpr const char *p2 = &str[1]; // both-error {{must be initialized by a constant expression}} \
+                                      // both-note {{cannot refer to element 1 of array of 0 elements in a constant expression}}
+}