[PATCH v4 2/3] stdlib: Implement mbrtoc8(), c8rtomb(), and the char8_t typedef.

[Tom Honermann]

Confirmed that this issue can be easily reproduced outside the testsuite.

$ cat t.cpp
#include <uchar.h>

$ g++ --version
g++ (GCC) 13.0.0 20220720 (experimental)
...

$ g++ -c -I/path/to/glibc-char8_t/include -std=c++17 
-Werror=c++20-compat t.cpp
In file included from t.cpp:1:
/home/tom/products/glibc-char8_t/include/uchar.h:38:23: error: 
identifier ‘char8_t’ is a keyword in C++20 [-Werror=c++20-compat]
    38 | typedef unsigned char char8_t;
       |                       ^~~~~~~
cc1plus: some warnings being treated as errors

The char8_t typedef is currently guarded by:

/* Declare the C2x char8_t typedef in C2x modes, but only if the C++
   __cpp_char8_t feature test macro is not defined.  */
#if __GLIBC_USE (ISOC2X) && !defined __cpp_char8_t
/* Define the 8-bit character type.  */
typedef unsigned char char8_t;
#endif

__GLIBC_USE (ISOC2X) evaluates to true because gcc unconditionally 
defines _GNU_SOURCE. I believe otherwise, C++17 mode would only (or 
should only) imply __GLIBC_USE (ISOC11).

Regardless, it seems that directives should be added to suppress the 
diagnostic. I tried prototyping such a fix, but it doesn't seem to work 
for me. I don't understand why.

$ diff -U3 uchar.h.old uchar.h
--- uchar.h.old 2022-07-20 12:37:55.544301692 -0400
+++ uchar.h     2022-07-20 12:43:21.124365563 -0400
@@ -34,8 +34,17 @@
  /* Declare the C2x char8_t typedef in C2x modes, but only if the C++
    __cpp_char8_t feature test macro is not defined.  */
  #if __GLIBC_USE (ISOC2X) && !defined __cpp_char8_t
+/* Suppress the C++20 compatability diagnostic regarding char8_t being a
+   keyword.  */
+#if defined __GNUC__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wc++20-compat"
+#endif
  /* Define the 8-bit character type.  */
  typedef unsigned char char8_t;
+#if defined __GNUC__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
+# pragma GCC diagnostic pop
+#endif
  #endif

  #ifndef __USE_ISOCXX11

Tom.

On 7/19/22 5:08 PM, Joseph Myers wrote:
> This change appears to introduce a failure of
> wcsmbs/check-installed-headers-cxx with GCC mainline, because uchar.h now
> produces:
>
> ../wcsmbs/uchar.h:38:23: error: identifier 'char8_t' is a keyword in C++20 [-Werror=c++20-compat]
>     38 | typedef unsigned char char8_t;
>
> (recall we want our installed headers to avoid warnings *without* relying
> on the default disabling of warnings in system headers).
>
> Unfortunately, GCC for C++ doesn't disable -Wc++20-compat inside
> __extension__ (unlike what the C front end does), so simply adding
> __extension__ to that declaration wouldn't help, but we could use
> diagnostic disabling pragmas (as already done in some installed headers).
>