string-desc: Avoid a gcc -Wshadow=compatible-local warning

[Bruno Haible via Gnulib discussion list]

When using an sd_substring expression as argument to an sd_last_index
invocation, I see this warning:

../gnulib-lib/string-desc.h:589:17: warning: declaration of '_s_' shadows a 
previous local [-Wshadow=compatible-local]
  589 |    ({typeof (s) _s_ = (s); \
      |                 ^~~
../gnulib-lib/string-desc.h:539:24: note: in definition of macro 'sd_last_index'
  539 |    ({typeof (s) _s_ = (s); \
      |                        ^

This patch fixes it. Not perfect, but good enough.


2026-10-31  Bruno Haible  <br...@clisp.org>

        string-desc: Avoid a gcc -Wshadow=compatible-local warning.
        * lib/string-desc.h (sd_substring): Rename local variable _s_ to _ss_.

diff --git a/lib/string-desc.h b/lib/string-desc.h
index 0e87c4d641..2ae167527c 100644
--- a/lib/string-desc.h
+++ b/lib/string-desc.h
@@ -586,15 +586,15 @@ extern rw_string_desc_t _rwsd_new_addr (idx_t n, 
/*!*/const/*!*/ char *addr)
 
 #if HAVE_RW_STRING_DESC
 # define sd_substring(s, start, end) \
-   ({typeof (s) _s_ = (s); \
+   ({typeof (s) _ss_ = (s); \
      idx_t _start_ = (start); \
      idx_t _end_ = (end); \
-     if (!(_start_ >= 0 && _start_ <= _end_ && _end_ <= _s_._nbytes)) \
+     if (!(_start_ >= 0 && _start_ <= _end_ && _end_ <= _ss_._nbytes)) \
        /* Invalid arguments.  */ \
        abort (); \
      typeof (s) _result_; \
      _result_._nbytes = _end_ - _start_; \
-     _result_._data = _s_._data + _start_; \
+     _result_._data = _ss_._data + _start_; \
      _result_; \
    })
 #else