[Bug c++/125333] Pack expansion of using-declaration of conversion operator 'taints' the type pack

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125333

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:244a5ba55006430db2dba1e2c58f7523ed01dbac

commit r16-9081-g244a5ba55006430db2dba1e2c58f7523ed01dbac
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed May 20 16:52:12 2026 -0400

    c++: ICE with variadic using-decl with conv fn [PR125284]

    When we have

      using foo<T>::operator T...;

    tsubst_decl/USING_DECL calls tsubst_pack_expansion twice and
    then builds up a USING_DECL via do_class_using_decl.  But we
    crash here with

      using foo<T>::operator T...;
      using foo<T>::operator T*...;

    because tsubst/TEMPLATE_TYPE_PARM gets an identifier node for
    __conv_op which is not TYPE_P.

    When we are substituting the first using, args is

     <tree_vec 0x7fffe99c6e20 length:1
        elt:0 <type_argument_pack 0x7fffe99e6690
            type <tree_vec 0x7fffe99c6e40 length:1 elt:0 <integer_type
0x7fffe98235e8 int>>
            VOID
            align:8 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7fffe99e6690>>

    and tsubst_pack_expansion gives us

     <tree_vec 0x7fffe99c6e40 length:1
        elt:0 <integer_type 0x7fffe98235e8 int public type_6 SI
            size <integer_cst 0x7fffe981b528 constant 32>
            unit-size <integer_cst 0x7fffe981b540 constant 4>
            align:32 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7fffe98235e8 precision:32 min <integer_cst 0x7fffe981b4e0 -2147483648> max
<integer_cst 0x7fffe981b4f8 2147483647>
            pointer_to_this <pointer_type 0x7fffe982ab28>>>

    Note that the tree_vecs are the same one.  So when we rewrite the
    arguments from tsubst_pack_expansion using make_conv_op_name, we
    also rewrite args.

            PR c++/125284
            PR c++/125333

    gcc/cp/ChangeLog:

            * pt.cc (tsubst_decl) <case USING_DECL>: Copy targs before
            modifying them.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/using-variadic4.C: New test.
            * g++.dg/cpp1z/using-variadic5.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit e28d144e3492b93b5b09e365439758e7403a16d2)