[FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

C11 provides static assertions via _Static_assert and
provides static_assert as a convenience define for this
in assert.h. MSVC 19.27 declares support for C11, but does
not support _Static_assert, but somehow supports
static_assert. That's therefore what we use.

But apparently there are some old GCC toolchains where
_Static_assert is supported, but assert.h does not provide
the fallback define. Some fate boxes are affected by this
[1].

This commit therefore checks whether static_assert works
with assert.h included; if not, it errors out. Users like
the above can still add -Dstatic_assert=_Static_assert
to cflags as a workaround.

[1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
This is what a test without fallback looks like.
Posted to gather opinions on what people prefer.

 configure | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/configure b/configure
index 6d7b33b0ff..c2d2c70c20 100755
--- a/configure
+++ b/configure
@@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
 check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
     { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
 
+test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
+#include <assert.h>
+#include <stddef.h>
+struct Foo {
+    int a;
+    void *ptr;
+} obj;
+static_assert(offsetof(struct Foo, a) == 0,
+              "First element of struct does not have offset 0");
+static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) + sizeof(obj.a),
+              "elements not properly ordered in struct");
+EOF
+
 check_cppflags -D_FILE_OFFSET_BITS=64
 check_cppflags -D_LARGEFILE_SOURCE
 
-- 
2.40.1

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