[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We could do this, and rely on the fact that the number of aliases is always
small so that we can claim it's O(1):

--- a/libstdc++-v3/include/std/text_encoding
+++ b/libstdc++-v3/include/std/text_encoding
@@ -644,6 +644,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __i;
     }

+    friend constexpr difference_type
+    operator-(_Sentinel __s, _Iterator __i)
+    {
+      __glibcxx_assert(__i._M_rep != nullptr);
+      difference_type __n = 0;
+      if (const _Rep* __rep = __i._M_rep)
+       while (__rep[__n]._M_id == __i._M_id)
+         ++__n;
+      return __n;
+    }
+
+    friend constexpr difference_type
+    operator-(_Iterator __i, _Sentinel __s)
+    { return -(__s - __i); }
+
   private:
     friend struct text_encoding;