$OpenBSD$

Index: runtime/druntime/src/core/stdc/wchar_.d
--- runtime/druntime/src/core/stdc/wchar_.d.orig
+++ runtime/druntime/src/core/stdc/wchar_.d
@@ -40,6 +40,16 @@ version( CRuntime_Glibc )
         ___value __value;
     }
 }
+else version (OpenBSD)
+{
+    // <sys/_types.h>
+    union __mbstate_t
+    {
+        char[128]   __mbstate8;
+        long        __mbstateL;
+    }
+    alias mbstate_t = __mbstate_t;
+}
 else version (Solaris)
 {
     ///
