copy-file: Silence gcc warnings

[Bruno Haible]

In GNU gettext, I see these compilation warnings with gcc 13.1.0:

gcc ... -Wall -Wno-cast-qual -Wno-conversion -Wno-float-equal -Wno-sign-compare 
-Wno-undef -Wno-unused-function -Wno-unused-parameter -Wno-float-conversion 
-Wimplicit-fallthrough -Wno-pedantic -Wno-sign-conversion -Wno-type-limits 
-Wno-unsuffixed-float-constants -g -O2 -c copy-file.c  -fPIC -DPIC -o 
.libs/libgettextlib_la-copy-file.o
copy-file.c: In function 'copy_file_preserving':
copy-file.c:192:7: warning: this statement may fall through 
[-Wimplicit-fallthrough=]
  192 |       error (EXIT_FAILURE, errno, _("error while opening %s for 
reading"),
      |       
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  193 |              quote (src_filename));
      |              ~~~~~~~~~~~~~~~~~~~~~
copy-file.c:195:5: note: here
  195 |     case GL_COPY_ERR_OPEN_BACKUP_WRITE:
      |     ^~~~
and so on.


The option -Wimplicit-fallthrough gets added as part of GL_CFLAG_GNULIB_WARNINGS
(collected by gl_CC_GNULIB_WARNINGS in m4/gnulib-common.m4). That explains
why I'm not seeing these warnings in gnulib testdirs.

It's probably too hard to teach gcc that error (EXIT_FAILURE, ...) does not
return but error (0, ...) does return. Therefore just disabling this warning
seems the best action.


2023-05-26  Bruno Haible  <bruno@clisp.org>

        copy-file: Silence gcc warnings.
        * lib/copy-file.c: Add #pragma GCC diagnostic.

diff --git a/lib/copy-file.c b/lib/copy-file.c
index 78da3996bb..bd98b35e7e 100644
--- a/lib/copy-file.c
+++ b/lib/copy-file.c
@@ -180,6 +180,13 @@ qcopy_file_preserving (const char *src_filename, const 
char *dest_filename)
   return err;
 }
 
+/* Silence gcc warnings "this statement may fall through".
+   gcc cannot know that error(), when invoked with a non-zero status argument,
+   will not return.  */
+#if __GNUC__ >= 7
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#endif
+
 void
 copy_file_preserving (const char *src_filename, const char *dest_filename)
 {