From 9494cbd4aadef3c34f7dccdc93d544b00c0a26b9 Mon Sep 17 00:00:00 2001
From: Bela Schaum <schaumb@users.noreply.github.com>
Date: Mon, 6 Mar 2023 13:02:46 +0000
Subject: [PATCH] Add a static assertion for issue
 https://github.com/Neargye/nameof/issues/52 (#57)

---
 include/nameof.hpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/nameof.hpp b/include/nameof.hpp
index 44230cc..f22cfdd 100644
--- a/include/nameof.hpp
+++ b/include/nameof.hpp
@@ -988,7 +988,13 @@ constexpr auto get_member_name() noexcept {
   if constexpr (std::is_member_function_pointer_v<decltype(V)>) {
     return n<V>();
   } else {
-    return n<V, &(union_type_holder<decltype(get_base_type(V))>::value.f.*V)>();
+    constexpr bool is_defined = sizeof(decltype(get_base_type(V))) != 0;
+    static_assert(is_defined, "Member name can use only if the struct is already fully defined. Please use NAMEOF macro, or separate definition and declaration.");
+    if constexpr (is_defined) {
+        return n<V, &(union_type_holder<decltype(get_base_type(V))>::value.f.*V)>();
+    } else {
+        return "";
+    }
   }
 }