Add check it type is complete to prefer_pass_by_value

In current implementation of `prefer_pass_by_value` we check sizeof type.
It fails in case that type is incomplete. This change adds a check if
there is a possibility to check the sizeof type.

Author: filipsajdak

include/cpp2util.h

@@ -491,9 +491,17 @@ template<typename T>
 //
 //-----------------------------------------------------------------------
 //
+template<typename, typename = void>
+constexpr bool is_complete_type_v = false;
+
+template<typename T>
+constexpr bool is_complete_type_v<T, std::void_t<decltype(sizeof(T))>> = true;
+
 template<typename T>
 constexpr bool prefer_pass_by_value =
-    sizeof(T) < 2*sizeof(void*) && std::is_trivially_copy_constructible_v<T>;
+    is_complete_type_v<T>
+    && sizeof(T) < 2*sizeof(void*)
+    && std::is_trivially_copy_constructible_v<T>;
 
 template<typename T>
 using in =