Re: [PATCH] c++: auto(x) partial substitution [PR110025, PR114138]

[Jason Merrill <jason@redhat.com>]

On 2/29/24 14:17, Patrick Palka wrote:
> On Wed, 28 Feb 2024, Jason Merrill wrote:
>> I wonder about, rather than returning it directly, setting its level to 1 for
>> the substitution?
> 
> Done, that works nicely.
> 
>> Then I wonder if it would be feasible to give all autos level 0 and adjust it
>> here?  That's probably not a stage 4 change, though...
> 
> It seems feasible.  I experimented doing this in the past[1] and ran
> into two complications.  One complication was with constrained auto
> deduction, e.g.
> 
>    template<class T>
>    void g() {
>      C<T*> auto x = ...;
>    };
> 
> Here the underlying concept-id that we enter satisfaction with is
> C<auto, T*> where this auto has level one greater than the template
> depth, and the argument vector we pass has an extra innermost level
> containing the deduced type, so things match up nicely.  This seems
> to be the only place where we truly need auto to have a non 0/1 level.
> In my WIP patch in that thread I just made do_auto_deduction build the
> concept-id C<auto, T*> in terms of an auto of the proper level before
> entering satisfaction, which was kind of ugly but worked.

So maybe set its level to TMPL_ARGS_DEPTH (targs) after 
add_to_template_args, rather than 1?

> The other complication was with Concepts TS extended auto deduction:
> 
>    tuple<auto...> t = tuple<int, char>{};
> 
> because unify_pack_expansion (called from fn_type_unification during
> do_auto_deduction) isn't prepared to see a parameter pack of level 0
> (unify has no problems with ordinary tparms of level 0 though).  This
> shouldn't be too hard to fix though.
> 
> How does the following look for trunk and perhaps 13 (there should be
> no functional change for code that doesn't use auto(x))?
> 
> [1]: https://gcc.gnu.org/pipermail/gcc-patches/2022-January/587818.html
> 
> -- >8 --
> 
> 	PR c++/110025
> 	PR c++/114138
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (make_cast_auto): Declare.
> 	* parser.cc (cp_parser_functional_cast): Replace a parsed auto
> 	with a level-less one via make_cast_auto.
> 	* pt.cc (find_parameter_packs_r): Don't treat level-less auto
> 	as a type parameter pack.
> 	(tsubst) <case TEMPLATE_TYPE_PARM>: Generalized CTAD placeholder
> 	handling to all level-less autos.
> 	(make_cast_auto): Define.
> 	(do_auto_deduction): Handle replacement of a level-less auto.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp23/auto-fncast16.C: New test.
> 	* g++.dg/cpp23/auto-fncast17.C: New test.
> 	* g++.dg/cpp23/auto-fncast18.C: New test.
> ---
>   gcc/cp/cp-tree.h                           |  1 +
>   gcc/cp/parser.cc                           | 11 ++++
>   gcc/cp/pt.cc                               | 37 +++++++++++-
>   gcc/testsuite/g++.dg/cpp23/auto-fncast16.C | 12 ++++
>   gcc/testsuite/g++.dg/cpp23/auto-fncast17.C | 15 +++++
>   gcc/testsuite/g++.dg/cpp23/auto-fncast18.C | 69 ++++++++++++++++++++++
>   6 files changed, 142 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast16.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast17.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast18.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 04c3aa6cd91..6f1da1c7bad 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7476,6 +7476,7 @@ extern tree make_decltype_auto			(void);
>   extern tree make_constrained_auto		(tree, tree);
>   extern tree make_constrained_decltype_auto	(tree, tree);
>   extern tree make_template_placeholder		(tree);
> +extern tree make_cast_auto			(void);
>   extern bool template_placeholder_p		(tree);
>   extern bool ctad_template_p			(tree);
>   extern bool unparenthesized_id_or_class_member_access_p (tree);
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index 3ee9d49fb8e..3dbe6722ba1 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -33314,6 +33314,17 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
>     if (!type)
>       type = error_mark_node;
>   
> +  if (TREE_CODE (type) == TYPE_DECL
> +      && is_auto (TREE_TYPE (type)))
> +    type = TREE_TYPE (type);
> +
> +  if (is_auto (type)
> +      && !AUTO_IS_DECLTYPE (type)
> +      && !PLACEHOLDER_TYPE_CONSTRAINTS (type)
> +      && !CLASS_PLACEHOLDER_TEMPLATE (type))
> +    /* auto(x) and auto{x} are represented by level-less auto.  */
> +    type = make_cast_auto ();
> +
>     if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
>       {
>         cp_lexer_set_source_position (parser->lexer);
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 2803824d11e..369e33f23c7 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -3921,7 +3921,8 @@ find_parameter_packs_r (tree *tp, int *walk_subtrees, void* data)
>   	 parameter pack (14.6.3), or the type-specifier-seq of a type-id that
>   	 is a pack expansion, the invented template parameter is a template
>   	 parameter pack.  */
> -      if (ppd->type_pack_expansion_p && is_auto (t))
> +      if (ppd->type_pack_expansion_p && is_auto (t)
> +	  && TEMPLATE_TYPE_LEVEL (t) != 0)
>   	TEMPLATE_TYPE_PARAMETER_PACK (t) = true;
>         if (TEMPLATE_TYPE_PARAMETER_PACK (t))
>           parameter_pack_p = true;
> @@ -16297,9 +16298,19 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>         }
>   
>       case TEMPLATE_TYPE_PARM:
> -      if (template_placeholder_p (t))
> +      if (TEMPLATE_TYPE_LEVEL (t) == 0)
>   	{
> +	  /* This is either an ordinary level-less auto or a CTAD placeholder
> +	     auto.  These get replaced only via do_auto_deduction which, in the
> +	     ordinary case, temporarily overrides its level to 1 before calling
> +	     tsubst.  CTAD placeholders are replaced via do_class_deduction.  */
> +	  gcc_checking_assert (is_auto (t));
>   	  tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (t);
> +	  if (!tmpl)
> +	    /* Ordinary level-less auto, nothing to substitute.  */
> +	    return t;
> +
> +	  /* Substitute the template of this CTAD placeholder.  */
>   	  tmpl = tsubst_expr (tmpl, args, complain, in_decl);
>   	  if (TREE_CODE (tmpl) == TEMPLATE_TEMPLATE_PARM)
>   	    tmpl = TEMPLATE_TEMPLATE_PARM_TEMPLATE_DECL (tmpl);
> @@ -29311,6 +29322,17 @@ template_placeholder_p (tree t)
>     return is_auto (t) && CLASS_PLACEHOLDER_TEMPLATE (t);
>   }
>   
> +/* Return an auto for an explicit cast, e.g. auto(x) or auto{x}.
> +   Like CTAD placeholders, these have level 0 so that they're not
> +   accidentally replaced via tsubst, and are always directly resolved
> +   via do_auto_deduction.  */
> +
> +tree
> +make_cast_auto ()
> +{
> +  return make_auto_1 (auto_identifier, true, /*level=*/0);
> +}
> +
>   /* Make a "constrained auto" type-specifier. This is an auto or
>     decltype(auto) type with constraints that must be associated after
>     deduction.  The constraint is formed from the given concept CON
> @@ -31213,7 +31235,16 @@ do_auto_deduction (tree type, tree init, tree auto_node,
>   	}
>       }
>   
> -  if (TEMPLATE_TYPE_LEVEL (auto_node) == 1)
> +  if (TEMPLATE_TYPE_LEVEL (auto_node) == 0)
> +    {
> +      /* Substitute this level-less auto via tsubst by temporarily
> +	 overriding its level to 1.  */
> +      TEMPLATE_TYPE_LEVEL (auto_node) = 1;
> +      type = tsubst (type, targs, complain, NULL_TREE);
> +      TEMPLATE_TYPE_LEVEL (auto_node) = 0;
> +      return type;
> +    }
> +  else if (TEMPLATE_TYPE_LEVEL (auto_node) == 1)
>       /* The outer template arguments are already substituted into type
>          (but we still may have used them for constraint checking above).  */;
>     else if (context == adc_unify)
> diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast16.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast16.C
> new file mode 100644
> index 00000000000..e2c13f6b050
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast16.C
> @@ -0,0 +1,12 @@
> +// PR c++/110025
> +// { dg-do compile { target c++23 } }
> +
> +template<auto V, class = decltype(auto(V)), class = decltype(auto{V})>
> +struct A { };
> +
> +template<auto V>
> +A<V> f();
> +
> +int main() {
> +  f<0>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast17.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast17.C
> new file mode 100644
> index 00000000000..25186dfdbf2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast17.C
> @@ -0,0 +1,15 @@
> +// PR c++/110025
> +// { dg-do compile { target c++23 } }
> +
> +template<class...> struct tuple;
> +
> +template<auto V>
> +using constant_t = int;
> +
> +template<auto... V>
> +using constants_t = tuple<constant_t<auto(V)>...>;
> +
> +using ty0 = constants_t<>;
> +using ty1 = constants_t<1>;
> +using ty2 = constants_t<1, 2>;
> +using ty3 = constants_t<1, 2, 3>;
> diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast18.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast18.C
> new file mode 100644
> index 00000000000..4656723684f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast18.C
> @@ -0,0 +1,69 @@
> +// PR c++/114138
> +// { dg-do compile { target c++23 } }
> +
> +namespace std {
> +  template <class T>
> +  T&& declval() noexcept requires true;
> +
> +  template <class>
> +  void declval() noexcept;
> +
> +  namespace detail {
> +    struct none_such;
> +    template <class>
> +    using none_such_t = none_such;
> +
> +    template <class T>
> +      extern const none_such_t<T> _getter_for;
> +
> +    template <class T>
> +    using _decay_t = decltype(auto(declval<T>()));
> +
> +    static_assert(__is_same_as(_decay_t<void>, void));
> +  }
> +
> +  template <const auto& Fn, class... Args>
> +    using _result_of_t = decltype(Fn(declval<Args>()...));
> +
> +  template <unsigned I, class Tuple>
> +    using tuple_element_t = _result_of_t<detail::_getter_for<detail::_decay_t<Tuple>>, char(*)[I+1], Tuple>;
> +
> +  template <class First, class Second>
> +  struct pair {
> +    First first;
> +    Second second;
> +  };
> +
> +  template <class>
> +    inline constexpr bool _is_pair = false;
> +
> +  template <class First, class Second>
> +    inline constexpr bool _is_pair<pair<First, Second>> = true;
> +
> +  template <class T>
> +    concept Pair = _is_pair<decltype(auto(std::declval<T>()))>;
> +
> +  template <unsigned I, Pair P>
> +    requires (I <= 1)
> +  decltype(auto) get(P&& p) noexcept {
> +    if constexpr (I == 0) {
> +      return (static_cast<P&&>(p).first);
> +    } else {
> +      return (static_cast<P&&>(p).second);
> +    }
> +  }
> +
> +  namespace detail {
> +    inline constexpr auto _pair_getter =
> +      []<unsigned J, class Pair>(char(*)[J], Pair&& p) noexcept -> decltype(auto) {
> +        return std::get<J-1>(static_cast<Pair&&>(p));
> +      };
> +
> +    template <class First, class Second>
> +      inline constexpr auto _getter_for<pair<First, Second>> = _pair_getter;
> +  }
> +
> +}
> +
> +static_assert(__is_same_as(int&, std::tuple_element_t<0, std::pair<int, float>&>));
> +static_assert(__is_same_as(float&&, std::tuple_element_t<1, std::pair<int, float>&&>));