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Subject: [Bug c++/103706] [11/12 Regression] ICE: tree check: accessed elt 1
 of 'tree_vec' with 0 elts in hash, at cp/constraint.cc:2503
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103706

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:db5f1c17031ad8a898d77121f1e0e0141306e22a

commit r12-7102-gdb5f1c17031ad8a898d77121f1e0e0141306e22a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Feb 8 08:46:32 2022 -0500

    c++: lambda in pack expansion using pack in constraint [PR103706]

    Here when expanding the pack expansion pattern containing a constrained
    lambda, the template argument for each Ts is an ARGUMENT_PACK_SELECT,
    which we store inside the lambda's LAMBDA_EXPR_REGEN_INFO.  Then during
    satisfaction of the lambda's constraint C<Ts> the satisfaction cache
    uses this argument as part of the key to the corresponding sat_entry, b=
ut
    iterative_hash_template_arg and template_args_equal deliberately don't
    handle ARGUMENT_PACK_SELECT.

    Since it's wrong to preserve ARGUMENT_PACK_SELECT inside a hash table
    due to its instability (as documented in iterative_hash_template_arg),
    this patch helps make sure the satisfaction cache doesn't see such trees
    by resolving ARGUMENT_PACK_SELECT arguments before adding them to
    LAMBDA_EXPR_REGEN_INFO.

            PR c++/103706

    gcc/cp/ChangeLog:

            * pt.cc (preserve_args): New function.
            (tsubst_lambda_expr): Use it when setting LAMBDA_EXPR_REGEN_INF=
O.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-lambda19.C: New test.=