[clang][bytecode] Handle non-arrays in initElem{,Pop} (llvm#112719)

... provided the given index is 0. Skip the atIndex() in that case.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.h

@@ -1863,27 +1863,48 @@ bool InitPop(InterpState &S, CodePtr OpPC) {
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool InitElem(InterpState &S, CodePtr OpPC, uint32_t Idx) {
   const T &Value = S.Stk.pop<T>();
-  const Pointer &Ptr = S.Stk.peek<Pointer>().atIndex(Idx);
+  const Pointer &Ptr = S.Stk.peek<Pointer>();
+
   if (Ptr.isUnknownSizeArray())
     return false;
-  if (!CheckInit(S, OpPC, Ptr))
+
+  // In the unlikely event that we're initializing the first item of
+  // a non-array, skip the atIndex().
+  if (Idx == 0 && !Ptr.getFieldDesc()->isArray()) {
+    Ptr.initialize();
+    new (&Ptr.deref<T>()) T(Value);
+    return true;
+  }
+
+  const Pointer &ElemPtr = Ptr.atIndex(Idx);
+  if (!CheckInit(S, OpPC, ElemPtr))
     return false;
-  Ptr.initialize();
-  new (&Ptr.deref<T>()) T(Value);
+  ElemPtr.initialize();
+  new (&ElemPtr.deref<T>()) T(Value);
   return true;
 }
 
 /// The same as InitElem, but pops the pointer as well.
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool InitElemPop(InterpState &S, CodePtr OpPC, uint32_t Idx) {
   const T &Value = S.Stk.pop<T>();
-  const Pointer &Ptr = S.Stk.pop<Pointer>().atIndex(Idx);
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
   if (Ptr.isUnknownSizeArray())
     return false;
-  if (!CheckInit(S, OpPC, Ptr))
+
+  // In the unlikely event that we're initializing the first item of
+  // a non-array, skip the atIndex().
+  if (Idx == 0 && !Ptr.getFieldDesc()->isArray()) {
+    Ptr.initialize();
+    new (&Ptr.deref<T>()) T(Value);
+    return true;
+  }
+
+  const Pointer &ElemPtr = Ptr.atIndex(Idx);
+  if (!CheckInit(S, OpPC, ElemPtr))
     return false;
-  Ptr.initialize();
-  new (&Ptr.deref<T>()) T(Value);
+  ElemPtr.initialize();
+  new (&ElemPtr.deref<T>()) T(Value);
   return true;
 }
 

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

@@ -52,6 +52,20 @@ consteval auto ok4() {
 }
 static_assert(ok4() == 37);
 
+consteval int ok5() {
+  int i;
+  new (&i) int[1]{1};
+
+  struct S {
+    int a; int b;
+  } s;
+  new (&s) S[1]{{12, 13}};
+
+  return 25;
+  // return s.a + s.b; FIXME: Broken in the current interpreter.
+}
+static_assert(ok5() == 25);
+
 /// FIXME: Broken in both interpreters.
 #if 0
 consteval int ok5() {