Re: [PATCH] c++/reflection: ^^ rejects pack-index-specifier [PR126546]

On 8/3/26 2:23 PM, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/16?
> 
> -- >8 --
> [expr.reflect] says that a reflection of a pack-index-expressions
> is ill-formed.  But that applies to the ^^id-expression production,
> not ^^type-id, in which a pack-index-specifier (for types) can be
> used.  So it's wrong for get_reflection to check PACK_INDEX_P, it
> should only reject PACK_INDEX_EXPR.
> 
> I don't think that currently we can get there with a PACK_INDEX_EXPR
> though: for a pack-index-expression Xs...[0] cp_parser_reflection_name
> will consume Xs and then immediately call get_reflection which gives
> an error.

This seems like a bit of a bug, since "A reflect-expression is parsed as 
the longest possible sequence of tokens that could syntactically form a 
reflect-expression.", i.e. the full pack-index-expression.  But not an 
important one, since it's ill-formed either way.

OK for trunk/16.

> But leaving the PACK_INDEX_EXPR check in doesn't seem like
> a bad idea.
> 
> 	PR c++/126546
> 
> gcc/cp/ChangeLog:
> 
> 	* reflect.cc (get_reflection): Check PACK_INDEX_EXPR instead of
> 	PACK_INDEX_P.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/reflect/pack-index1.C: Accept a reflection of a
> 	pack-index-specifier.  Reject a reflection of a
> 	pack-index-expression.
> 	* g++.dg/reflect/pack-index2.C: New test.
> ---
>   gcc/cp/reflect.cc                          | 2 +-
>   gcc/testsuite/g++.dg/reflect/pack-index1.C | 9 ++++++++-
>   gcc/testsuite/g++.dg/reflect/pack-index2.C | 8 ++++++++
>   3 files changed, 17 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/reflect/pack-index2.C
> 
> diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
> index 989d7cebc77..7b4142f2d4f 100644
> --- a/gcc/cp/reflect.cc
> +++ b/gcc/cp/reflect.cc
> @@ -141,7 +141,7 @@ get_reflection (location_t loc, tree t, reflect_kind kind/*=REFLECT_UNDEF*/)
>   
>     /* Constant template parameters and pack-index-expressions cannot
>        appear as operands of the reflection operator.  */
> -  if (PACK_INDEX_P (t))
> +  if (TREE_CODE (t) == PACK_INDEX_EXPR)
>       {
>         error_at (loc, "%<^^%> cannot be applied to a pack index");
>         return error_mark_node;
> diff --git a/gcc/testsuite/g++.dg/reflect/pack-index1.C b/gcc/testsuite/g++.dg/reflect/pack-index1.C
> index f68705b1299..c2b27a8c133 100644
> --- a/gcc/testsuite/g++.dg/reflect/pack-index1.C
> +++ b/gcc/testsuite/g++.dg/reflect/pack-index1.C
> @@ -14,10 +14,16 @@ f ()
>   {
>     constexpr auto r1 = ^^Ts...[0]::sx;
>     constexpr auto r2 = ^^typename Ts...[0]::type;
> +  constexpr auto e = ^^Ts...[0];
> +}
>   
> +template<int... Ns>
> +void
> +g ()
> +{
>     // NTTPs and pack-index-expressions cannot appear as operands
>     // of the reflection operator.
> -  constexpr auto e = ^^Ts...[0]; // { dg-error "cannot be applied" }
> +  constexpr auto e = ^^Ns...[0];  // { dg-error "cannot be applied" }
>   }
>   
>   
> @@ -25,4 +31,5 @@ void
>   g ()
>   {
>     f<S>();
> +  g<1, 2, 3>();
>   }
> diff --git a/gcc/testsuite/g++.dg/reflect/pack-index2.C b/gcc/testsuite/g++.dg/reflect/pack-index2.C
> new file mode 100644
> index 00000000000..113629510f1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/reflect/pack-index2.C
> @@ -0,0 +1,8 @@
> +// PR c++/126546
> +// { dg-do compile { target c++26 } }
> +// { dg-additional-options "-freflection" }
> +
> +template <class... Ts>
> +consteval auto first() { return ^^Ts...[0]; }
> +
> +static_assert(first<int, double>() == ^^int);
> 
> base-commit: 71d225bf68f1c5cf31b2e7fd5759b43f8dadc280