Re: [PATCH] libstdc++: LWG 3301 transform_view::iterator has incorrect iterator_category

[Patrick Palka <ppalka@redhat.com>]

On Mon, 24 Feb 2020, Patrick Palka wrote:

> libstdc++-v3/ChangeLog:
> 
> 	LWG 3301 transform_view::_Iterator has incorrect iterator_category
> 	* include/std/ranges (transform_view::_Iterator::_S_iter_cat): Adjust
> 	determination of iterator_category as per LWG 3301.
> 	* testsuite/std/ranges/adaptors/transform.cc: Augment test.
> ---
>  libstdc++-v3/include/std/ranges               | 16 +++++++++----
>  .../std/ranges/adaptors/transform.cc          | 24 +++++++++++++++++++
>  2 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index ab8fbaca38f..aed90e9710e 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -1570,12 +1570,18 @@ namespace views
>  	  static constexpr auto
>  	  _S_iter_cat()
>  	  {
> -	    using _Cat
> -              = typename iterator_traits<_Base_iter>::iterator_category;
> -	    if constexpr (derived_from<_Cat, contiguous_iterator_tag>)
> -	      return random_access_iterator_tag{};
> +	    using _Res = invoke_result_t<_Fp&, range_reference_t<_Base>>;

Consider this line fixed to use 'typename'.

> +	    if constexpr (is_lvalue_reference_v<_Res>)
> +	      {
> +		using _Cat
> +		  = typename iterator_traits<_Base_iter>::iterator_category;
> +		if constexpr (derived_from<_Cat, contiguous_iterator_tag>)
> +		  return random_access_iterator_tag{};
> +		else
> +		  return _Cat{};
> +	      }
>  	    else
> -	      return _Cat{};
> +	      return input_iterator_tag{};
>  	  }
>  
>  	  static constexpr decltype(auto)
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
> index ad51fffb43d..0845febe2cf 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
> @@ -77,10 +77,34 @@ test03()
>    VERIFY( ranges::equal(v, (int[]){1,2,3,4,5}) );
>  }
>  
> +void
> +test04()
> +{
> +  // LWG 3302

Consider this comment fixed to say 3301.