[clang][Interp] Fix assertion in InitElem{,Pop} ops

... when the pointer is an unknown size array.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1448,6 +1448,8 @@ 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);
+  if (Ptr.isUnknownSizeArray())
+    return false;
   if (!CheckInit(S, OpPC, Ptr))
     return false;
   Ptr.initialize();
@@ -1460,6 +1462,8 @@ 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);
+  if (Ptr.isUnknownSizeArray())
+    return false;
   if (!CheckInit(S, OpPC, Ptr))
     return false;
   Ptr.initialize();

clang/test/AST/Interp/arrays.cpp

@@ -138,6 +138,8 @@ constexpr int dependent[4] = {
 static_assert(dependent[2] == dependent[0], "");
 static_assert(dependent[3] == dependent[1], "");
 
+union { char x[]; } r = {0};
+
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wc99-extensions"
 #pragma clang diagnostic ignored "-Winitializer-overrides"