GHC 9.4.4 `Rts.h` fails to compile after `rts/PosixSource.h` on FreeBSD 12.3

Summary

As of commit d2ea5bfb Rts.h attempts to use static_assert():

+/* For _Static_assert */
+#include <assert.h>
...
+#if __STDC_VERSION__ >= 201112L
+#define GHC_STATIC_ASSERT(x, msg) static_assert((x), msg)
+#else
+#define GHC_STATIC_ASSERT(x, msg)
+#endif

But in code generated from derivedConstants the order of #include files is:

#include "rts/PosixSource.h"
#include "Rts.h"
...

which defines _POSIX_SOURCE on FreeBSD systems, which blocks the visibility of the C23 static_assert() (only the C11 original _Static_assert() is defined). The <assert.h> file on FreeBSD 12 reads in part:

/*
 * Static assertions.  In principle we could define static_assert for
 * C++ older than C++11, but this breaks if _Static_assert is
 * implemented as a macro.
 * 
 * C++ template parameters may contain commas, even if not enclosed in
 * parentheses, causing the _Static_assert macro to be invoked with more
 * than two parameters.
 */
#if __ISO_C_VISIBLE >= 2011 && !defined(__cplusplus)
#define static_assert   _Static_assert
#endif

has static_assert conditional on __ISO_C_VISIBLE, and that macro is hidden when _POSIX_SOURCE is defined. From <sys/cdefs.h>:

#if defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE)
# define _POSIX_C_SOURCE         198808
#endif
#ifdef _POSIX_C_SOURCE
# if _POSIX_C_SOURCE >= 200809
#  define __POSIX_VISIBLE         200809
#  define __ISO_C_VISIBLE         1999
# elif /* ... various older variants ... */
# ...
# endif /* _POSIX_C_SOURCE */
#else
/*-
 * Deal with _ANSI_SOURCE:
 ...
 */
# if defined(_ANSI_SOURCE)       /* Hide almost everything. */
#  define __POSIX_VISIBLE         0
#  define __XSI_VISIBLE           0
#  define __BSD_VISIBLE           0
#  define __ISO_C_VISIBLE         1990
#  define __EXT1_VISIBLE          0
# elif defined(_C99_SOURCE)      /* Localism to specify strict C99 env. */
#  define __POSIX_VISIBLE         0
#  define __XSI_VISIBLE           0
#  define __BSD_VISIBLE           0
#  define __ISO_C_VISIBLE         1999
#  define __EXT1_VISIBLE          0
# elif defined(_C11_SOURCE)      /* Localism to specify strict C11 env. */
#  define __POSIX_VISIBLE         0
#  define __XSI_VISIBLE           0
#  define __BSD_VISIBLE           0
#  define __ISO_C_VISIBLE         2011
#  define __EXT1_VISIBLE          0
# else                           /* Default environment: show everything. */
#  define __POSIX_VISIBLE         200809
#  define __XSI_VISIBLE           700
#  define __BSD_VISIBLE           1
#  define __ISO_C_VISIBLE         2011
#  define __EXT1_VISIBLE          1
# endif
#endif

Bottom line __ISO_C_VISIBLE is set to 2011 only if none of the various standards-compliance _FOO_SOURCE macros are defined. As a result the code generated by deriveConstants fails to compile:

Command line: .../deriveConstants --gen-header -o .../stage1/rts/build/include/DerivedConstants.h --tmpdir /tmp/extra-dir-51880349607072 --gcc-program /usr/local/bin/gcc10 --gcc-flag -Wall --gcc-flag -Werror=unused-but-set-variable --gcc-flag -Wno-error=inline --gcc-flag -Irts --gcc-flag -Irts/include --gcc-flag -I.../stage1/rts/build/include --gcc-flag -fcommon --nm-program /usr/local/bin/nm --objdump-program /usr/local/bin/objdump --target-os freebsd
===> Command failed with error code: 1
In file included from /tmp/extra-dir-51880349607072/tmp.c:14:
rts/include/Rts.h:170:49: error: expected declaration specifiers or '...' before '(' token
  170 | #define GHC_STATIC_ASSERT(x, msg) static_assert((x), msg)
      |                                                 ^

Possible work-arounds:

• Continue to use _Static_assert.
• Include <assert.h> first.
• Avoid the static assertion on FreeBSD 12

Steps to reproduce

Attempt to build GHC 9.4.4 on FreeBSD 12.3.

Expected behavior

GHC 9.4.4 should build

Environment

• GHC version used: 9.4.4

Optional:

• Operating System: FreeBSD 12.3
• System Architecture: x86_64