[all-commits] [llvm/llvm-project] 1bbf3a: [libc++] Fix `reverse_iterator` when underlying is...

[Hui via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1bbf3a37056761ec407031431e28f856428566f0
      https://github.com/llvm/llvm-project/commit/1bbf3a37056761ec407031431e28f856428566f0
  Author: Hui <hui.xie1990 at gmail.com>
  Date:   2024-10-19 (Sat, 19 Oct 2024)

  Changed paths:
    M libcxx/include/__iterator/reverse_iterator.h
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/equal.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/greater-equal.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/greater.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/less-equal.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/less.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/not-equal.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/assign.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.default.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.iter.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.reverse_iterator.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.conv/base.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.elem/arrow.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.elem/bracket.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.elem/dereference.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/decrement-assign.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/increment-assign.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/minus.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/plus.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/postdecrement.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/postincrement.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/predecrement.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nav/preincrement.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nonmember/make_reverse_iterator.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nonmember/minus.pass.cpp
    M libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nonmember/plus.pass.cpp

  Log Message:
  -----------
  [libc++] Fix `reverse_iterator` when underlying is c++20 `bidirectional_iterator` but not `Cpp17BidirectionalIterator` (#112100)

`reverse_iterator` supports either c++20 `bidirectional_iterator` or
`Cpp17BidirectionalIterator `
http://eel.is/c++draft/reverse.iter.requirements

The current `reverse_iterator` uses `std::prev` in its `operator->`,
which only supports the `Cpp17BidirectionalIterator` properly.

If the underlying iterator is c++20 `bidirectional_iterator` but does
not satisfy the named requirement `Cpp17BidirectionalIterator`,
(examples are `zip_view::iterator`, `flat_map::iterator`), the current
`std::prev` silently compiles but does a no-op and returns the same
iterator back. So `reverse_iterator::operator->` will silently give a
wrong answer.

Even if we fix the behaviour of `std::prev`, at best, we could fail to
compile the code. But this is not ok, because we need to support this
kind of iterators in `reverse_iterator`.

The solution is simply to not use `std::prev`.

---------

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>



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