[clang][bytecode] Fix return value of array CXXNewExprs (#127526)

Just like with the __builtin_operator_new version, we need to point to
the first array element, not the array element itself.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -1063,7 +1063,8 @@ bool Free(InterpState &S, CodePtr OpPC, bool DeleteIsArrayForm,
       return false;
     }
 
-    if (!Ptr.isRoot() || Ptr.isOnePastEnd() || Ptr.isArrayElement()) {
+    if (!Ptr.isRoot() || Ptr.isOnePastEnd() ||
+        (Ptr.isArrayElement() && Ptr.getIndex() != 0)) {
       const SourceInfo &Loc = S.Current->getSource(OpPC);
       S.FFDiag(Loc, diag::note_constexpr_delete_subobject)
           << Ptr.toDiagnosticString(S.getASTContext()) << Ptr.isOnePastEnd();

clang/lib/AST/ByteCode/Interp.h

@@ -2915,13 +2915,17 @@ inline bool AllocN(InterpState &S, CodePtr OpPC, PrimType T, const Expr *Source,
     S.Stk.push<Pointer>(0, nullptr);
     return true;
   }
+  assert(NumElements.isPositive());
 
   DynamicAllocator &Allocator = S.getAllocator();
   Block *B =
       Allocator.allocate(Source, T, static_cast<size_t>(NumElements),
                          S.Ctx.getEvalID(), DynamicAllocator::Form::Array);
   assert(B);
-  S.Stk.push<Pointer>(B);
+  if (NumElements.isZero())
+    S.Stk.push<Pointer>(B);
+  else
+    S.Stk.push<Pointer>(Pointer(B).atIndex(0));
   return true;
 }
 
@@ -2941,13 +2945,18 @@ inline bool AllocCN(InterpState &S, CodePtr OpPC, const Descriptor *ElementDesc,
     S.Stk.push<Pointer>(0, ElementDesc);
     return true;
   }
+  assert(NumElements.isPositive());
 
   DynamicAllocator &Allocator = S.getAllocator();
   Block *B =
       Allocator.allocate(ElementDesc, static_cast<size_t>(NumElements),
                          S.Ctx.getEvalID(), DynamicAllocator::Form::Array);
   assert(B);
-  S.Stk.push<Pointer>(B);
+  if (NumElements.isZero())
+    S.Stk.push<Pointer>(B);
+  else
+    S.Stk.push<Pointer>(Pointer(B).atIndex(0));
+
   return true;
 }
 

clang/test/AST/ByteCode/new-delete.cpp

@@ -922,6 +922,20 @@ namespace NonConstexprArrayCtor {
                          // both-note {{in call to}}
 }
 
+namespace ArrayBaseCast {
+  struct A {};
+  struct B : A {};
+  constexpr bool test() {
+    B *b = new B[2];
+
+    A* a = b;
+
+    delete[] b;
+    return true;
+  }
+  static_assert(test());
+}
+
 #else
 /// Make sure we reject this prior to C++20
 constexpr int a() { // both-error {{never produces a constant expression}}