[clang][Interp] Use an array root's field decl in the LValuePath

Instead of pushing the index 0.

Author: tbaederr

clang/lib/AST/Interp/Pointer.cpp

@@ -152,8 +152,9 @@ APValue Pointer::toAPValue() const {
   Pointer Ptr = *this;
   while (Ptr.isField() || Ptr.isArrayElement()) {
     if (Ptr.isArrayRoot()) {
-        Path.push_back(APValue::LValuePathEntry::ArrayIndex(0));
-        Ptr = Ptr.getBase();
+      Path.push_back(APValue::LValuePathEntry(
+          {Ptr.getFieldDesc()->asDecl(), /*IsVirtual=*/false}));
+      Ptr = Ptr.getBase();
     } else if (Ptr.isArrayElement()) {
       if (Ptr.isOnePastEnd())
         Path.push_back(APValue::LValuePathEntry::ArrayIndex(Ptr.getArray().getNumElems()));

clang/test/AST/Interp/functions.cpp

@@ -644,3 +644,21 @@ namespace FunctionCast {
                              // both-warning {{are a Clang extension}}
   int b[(int)IntFn(f)()];    // ok
 }
+
+#if __cplusplus >= 202002L
+namespace StableAddress {
+  template<unsigned N> struct str {
+    char arr[N];
+  };
+  // FIXME: Deduction guide not needed with P1816R0.
+  template<unsigned N> str(const char (&)[N]) -> str<N>;
+
+  template<str s> constexpr int sum() {
+    int n = 0;
+    for (char c : s.arr)
+      n += c;
+    return n;
+  }
+  static_assert(sum<str{"$hello $world."}>() == 1234, "");
+}
+#endif