[PATCH 2/2] c++: maybe_dependent_member_ref and typenames [PR118626]

[Jason Merrill]

On 3/31/25 7:51 PM, Patrick Palka wrote:
> Here during maybe_dependent_member_ref for accepted_type<_Up>, we
> correctly don't strip the typedef because it's a complex one (its
> defaulted template parameter isn't used in its definition) and so
> we recurse to consider its corresponding TYPE_DECL.
> 
> We then incorrectly decide to not rewrite this use because of the
> TYPENAME_TYPE shortcut.  But I don't think this shortcut should apply to
> a typedef TYPE_DECL.
> 
> 	PR c++/118626
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (maybe_dependent_member_ref): Restrict TYPENAME_TYPE
> 	shortcut to non-typedef TYPE_DECL.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/class-deduction-alias25a.C: New test.
> ---
>   gcc/cp/pt.cc                                  |  3 ++-
>   .../g++.dg/cpp2a/class-deduction-alias25a.C   | 19 +++++++++++++++++++
>   2 files changed, 21 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 6509089efdc..a1b0df428dc 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -17786,7 +17786,8 @@ maybe_dependent_member_ref (tree t, tree args, tsubst_flags_t complain,
>   
>     if (TREE_CODE (t) == TYPE_DECL)
>       {
> -      if (TREE_CODE (TREE_TYPE (t)) == TYPENAME_TYPE
> +      if (!DECL_ORIGINAL_TYPE (t)

I think let's check !is_typedef_decl instead.  OK with that tweak.

> +	  && TREE_CODE (TREE_TYPE (t)) == TYPENAME_TYPE
>   	  && TYPE_NAME (TREE_TYPE (t)) == t)
>   	/* The TYPE_DECL for a typename has DECL_CONTEXT of the typename
>   	   scope, but it doesn't need to be rewritten again.  */
> diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C
> new file mode 100644
> index 00000000000..74ef1e4d890
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C
> @@ -0,0 +1,19 @@
> +// PR c++/118626
> +// { dg-do compile { target c++20 } }
> +
> +template<long> struct _Nth_type { using type = _Nth_type; };
> +
> +template<class _Up>
> +struct variant {
> +  template<class _Tp> static constexpr long __accepted_index = 0;
> +  template<long _Np> using __to_type = typename _Nth_type<_Np>::type;
> +  template<class _Tp, int = sizeof(_Tp)> using __accepted_type = __to_type<__accepted_index<_Tp>>;
> +  template<class = __accepted_type<_Up>> variant(_Up);
> +};
> +
> +template<class _Tp>
> +struct Node { Node(_Tp); };
> +
> +template<class R> using Tree = variant<Node<R>>;
> +using type = decltype(Tree{Node{42}});
> +using type = Tree<int>;