feat(headers): stddef/stdint/stdbool/limits/float/assert + features wiring

include/stddef.h

@@ -20,11 +20,15 @@ typedef __SIZE_TYPE__ size_t;
 /* Signed integer type of the difference of two pointers. */
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
 
-/* Wide character type. */
+/* Wide character type; a C++ keyword there, so no redefinition. */
+#ifndef __cplusplus
 typedef __WCHAR_TYPE__ wchar_t;
+#endif
 
-/* Type of the null pointer constant nullptr (C23). */
+/* Type of the null pointer constant nullptr (C23); a C++ keyword there. */
+#if !defined(__cplusplus) && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 typedef typeof(nullptr) nullptr_t;
+#endif
 
 /* Null pointer constant. */
 #define NULL ((void *)0)
@@ -32,6 +36,11 @@ typedef typeof(nullptr) nullptr_t;
 /* Offset in bytes of a member from the start of its enclosing object. */
 #define offsetof(type, m) __builtin_offsetof(type, m)
 
+/* Mark the following point of the program as unreachable (C23). */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define unreachable() __builtin_unreachable()
+#endif
+
 /*
  * An object type whose alignment is as great as that of any supported
  * fundamental type; suitable as the storage type for aligned allocation.