diff --git a/internal.h b/internal.h
index 59f5d640cc..f6742cd458 100644
--- a/internal.h
+++ b/internal.h
@@ -2066,6 +2066,19 @@ rb_obj_builtin_type(VALUE obj)
 # define FLEX_ARY_LEN 1 /* VALUE ary[1]; */
 #endif
 
+/*
+ * For declaring bitfields out of non-unsigned int types:
+ *   struct date {
+ *      BITFIELD(enum months) month:4;
+ *      ...
+ *   };
+ */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define BITFIELD(type) type
+#else
+# define BITFIELD(type) unsigned int
+#endif
+
 #if defined(__cplusplus)
 #if 0
 { /* satisfy cc-mode */
diff --git a/method.h b/method.h
index 83f3acac06..e2d121d9b8 100644
--- a/method.h
+++ b/method.h
@@ -32,16 +32,10 @@ typedef enum {
     METHOD_VISI_MASK = 0x03
 } rb_method_visibility_t;
 
-#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
-#define bits_t rb_method_visibility_t
-#else
-#define bits_t unsigned int
-#endif
 typedef struct rb_scope_visi_struct {
-    bits_t method_visi : 3;
+    BITFIELD(rb_method_visibility_t) method_visi : 3;
     unsigned int module_func : 1;
 } rb_scope_visibility_t;
-#undef bits_t
 
 /*! CREF (Class REFerence) */
 typedef struct rb_cref_struct {