libstdc++: Add noexcept to some std::counted_iterator operations

This was inspired by LWG 4245 but goes further. Anything which only
reads or writes the _M_length member can be noexcept. That
member is an iterator difference_type which means it's a signed integer
type or the __max_diff_type integer-like class type, so all arithmetic
and comparisons are non-throwing.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (counted_iterator): Add noexcept
	to friend operators which only access the _M_length member.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index bed72955d0c41392c935d040c3cc231e91024920..478a98fe8a4f36a3ee919ac5077a00a0ebd1581c 100644 (file)
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -2511,17 +2511,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        [[nodiscard]]
        friend constexpr iter_difference_t<_It2>
        operator-(const counted_iterator& __x,
-                 const counted_iterator<_It2>& __y)
+                 const counted_iterator<_It2>& __y) noexcept
        { return __y._M_length - __x._M_length; }
 
       [[nodiscard]]
       friend constexpr iter_difference_t<_It>
-      operator-(const counted_iterator& __x, default_sentinel_t)
+      operator-(const counted_iterator& __x, default_sentinel_t) noexcept
       { return -__x._M_length; }
 
       [[nodiscard]]
       friend constexpr iter_difference_t<_It>
-      operator-(default_sentinel_t, const counted_iterator& __y)
+      operator-(default_sentinel_t, const counted_iterator& __y) noexcept
       { return __y._M_length; }
 
       constexpr counted_iterator&
@@ -2548,19 +2548,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        [[nodiscard]]
        friend constexpr bool
        operator==(const counted_iterator& __x,
-                  const counted_iterator<_It2>& __y)
+                  const counted_iterator<_It2>& __y) noexcept
        { return __x._M_length == __y._M_length; }
 
       [[nodiscard]]
       friend constexpr bool
-      operator==(const counted_iterator& __x, default_sentinel_t)
+      operator==(const counted_iterator& __x, default_sentinel_t) noexcept
       { return __x._M_length == 0; }
 
       template<common_with<_It> _It2>
        [[nodiscard]]
        friend constexpr strong_ordering
        operator<=>(const counted_iterator& __x,
-                   const counted_iterator<_It2>& __y)
+                   const counted_iterator<_It2>& __y) noexcept
        { return __y._M_length <=> __x._M_length; }
 
       [[nodiscard]]