[gcc r14-10919] libstdc++: Fix typos in iterator increment for std::text_encoding [PR117520]

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:17ff72651dbba6790ce4c492aa2df78f1dbdaf3e

commit r14-10919-g17ff72651dbba6790ce4c492aa2df78f1dbdaf3e
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Nov 11 11:54:00 2024 +0000

    libstdc++: Fix typos in iterator increment for std::text_encoding [PR117520]
    
    The intended behaviour for std::text_encoding::aliases_view's iterator
    is that it incrementing or decrementing too far sets it to a
    value-initialized state, or fails an assertion when those are enabled.
    There were typos that used == instead of = which meant that instead of
    becoming singular or aborting, an out-of-range increment just did
    nothing. This meant erroneous operations were well-defined and didn't
    produce any undefined behaviour, but were not diagnosed with assertions
    enabled, as had been intended.
    
    This change fixes the bugs and adds more tests to verify the intended
    behaviour.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/117520
            * include/std/text_encoding (aliases_view:_Iterator::operator+=):
            Fix typos that caused == to be used instead of =.
            (aliases_view::_Iterator): Fix friend declaration.
            * testsuite/std/text_encoding/members.cc: Adjust expected
            behaviour of invalid subscript. Add tests for other erroneous
            operations on iterators.

Diff:
---
 libstdc++-v3/include/std/text_encoding              |  6 +++---
 libstdc++-v3/testsuite/std/text_encoding/members.cc | 19 +++++++++++++++++--
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/text_encoding 
b/libstdc++-v3/include/std/text_encoding
index 83d023bc71bd..515f6e52345f 100644
--- a/libstdc++-v3/include/std/text_encoding
+++ b/libstdc++-v3/include/std/text_encoding
@@ -573,7 +573,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                          && _M_rep[__n - 1]._M_id == _M_id) [[likely]]
                 _M_rep += __n;
              else
-               *this == _Iterator{};
+               *this = _Iterator{};
            }
          else if (__n < 0)
            {
@@ -581,7 +581,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                    && _M_rep[__n]._M_id == _M_id) [[likely]]
                 _M_rep += __n;
              else
-               *this == _Iterator{};
+               *this = _Iterator{};
            }
        }
       if (__n != 0)
@@ -643,7 +643,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   private:
-    friend class text_encoding;
+    friend struct text_encoding;
 
     constexpr explicit
     _Iterator(const _Rep* __r) noexcept
diff --git a/libstdc++-v3/testsuite/std/text_encoding/members.cc 
b/libstdc++-v3/testsuite/std/text_encoding/members.cc
index adbd74ab85ea..253653250c8a 100644
--- a/libstdc++-v3/testsuite/std/text_encoding/members.cc
+++ b/libstdc++-v3/testsuite/std/text_encoding/members.cc
@@ -70,10 +70,25 @@ test_every_id()
     auto end = aliases.end();
     VERIFY( (begin + std::ranges::distance(aliases)) == end );
 #ifndef _GLIBCXX_ASSERTIONS
-    // This is an error, but with assertions disabled is guaranteed safe:
+    // These ops violate preconditions, but as libstdc++ extensions they are
+    // guaranteed to either assert or have well-defined behaviour.
+
+    // This erroneously returns ""sv:
     VERIFY( begin[std::ranges::distance(aliases)] == ""sv );
     // Likewise:
-    VERIFY( begin[999999] == *begin );
+    VERIFY( begin[999999] == ""sv );
+
+    auto iter = begin;
+    std::ranges::advance(iter, end);
+    // Erroneously sets iter to a value-initialized state.
+    ++iter;
+    VERIFY( iter == decltype(iter){} );
+    VERIFY( *iter == ""sv );
+
+    iter = begin;
+    // Erroneously sets iter to a value-initialized state.
+    --iter;
+    VERIFY( iter == decltype(iter){} );
 #endif
   }
 }