[cfe-dev] -pedantic warnings in system headers?

[Stephan Bergmann via cfe-dev]

The assert.h in glibc 2.25 defines assert as a GNU-extension "statement 
expression", see 
<https://sourceware.org/git/?p=glibc.git;a=commit;h=e077349ce589466eecd47213db4fae6b80ec18c4> 
"assert.h: allow gcc to detect assert(a = 1) errors":

> /* When possible, define assert so that it does not add extra
>    parentheses around EXPR.  Otherwise, those added parentheses would
>    suppress warnings we'd expect to be detected by gcc's -Wparentheses.  */
> # if !defined __GNUC__ || defined __STRICT_ANSI__
> #  define assert(expr)                                                 \
>     ((expr)                                                            \
>      ? __ASSERT_VOID_CAST (0)                                          \
>      : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
> # else
> #  define assert(expr)                                                 \
>     ({                                                                 \
>       if (expr)                                                                \
>         ; /* empty */                                                  \
>       else                                                             \
>         __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION);  \
>     })
> # endif

A simple program

> $ cat test.c
> #include <assert.h>
> int main() {
>     assert(1);
> }

causes no issues when compiled with GCC (some random "gcc (GCC) 7.1.1 
20170622 (Red Hat 7.1.1-3)"),

> $ gcc -pedantic test.c

but emits a diagnostic when compiled with Clang (at least recent trunk),

> $ /data/llvm/inst/bin/clang -pedantic test.c
> test.c:3:5: warning: use of GNU statement expression extension [-Wgnu-statement-expression]
>     assert(1);
>     ^
> /usr/include/assert.h:95:6: note: expanded from macro 'assert'
>     ({                                                                  \
>      ^
> 1 warning generated.

 From a quick look at the code of both GCC and Clang, I assume the 
difference is due to GCC implicitly assuming /usr/include/assert.h to be 
a system header (so it suppresses the pedantic warning there), while 
Clang does not.  (Curiously, while many of the libstdc++ files in 
/usr/include/c++/... have

> #pragma GCC system_header

near the top, none of the files from glibc in /usr/include/ do.)

Anybody got any further information, whether the difference in behavior 
is by accident or by design?