[Bug c++/103868] ICE at end of coroutine when using asio

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103868

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Iain D Sandoe <iains@gcc.gnu.org>:

https://gcc.gnu.org/g:9cb1f565a91e2dd57098c43593954b57c065a19b

commit r12-8307-g9cb1f565a91e2dd57098c43593954b57c065a19b
Author: Nathan Sidwell <nathan@acm.org>
Date:   Sun Apr 3 11:35:03 2022 +0100

    c++, coroutines: Avoid expanding within templates [PR103868]

    This is a forward-port of a patch by Nathan (against 10.x) which fixes an
open
    PR.

    We are ICEing because we ended up tsubst_copying something that had already
    been tsubst, leading to an assert failure (mostly such repeated tsubsting
is
    harmless).

    We had a non-dependent co_await in a non-dependent-type template fn, so we
    processed it at definition time, and then reprocessed at instantiation
time.
    We fix this here by deferring substitution while processing templates.

    Additional observations (for a better future fix, in the GCC13 timescale):

    Exprs only have dependent type if at least one operand is dependent which
was
    what the current code was intending to do.  Coroutines have the additional
    wrinkle, that the current fn's type is an implicit operand.

    So, if the coroutine function's type is not dependent, and the operand is
not
    dependent, we should determine the type of the co_await expression using
the
    DEPENDENT_EXPR wrapper machinery.  That allows us to determine the
    subexpression type, but leave its operand unchanged and then instantiate it
    later.

            PR c++/103868

    gcc/cp/ChangeLog:

            * coroutines.cc (finish_co_await_expr): Do not process
non-dependent
            coroutine expressions at template definition time.
            (finish_co_yield_expr): Likewise.
            (finish_co_return_stmt): Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/coroutines/pr103868.C: New test.

    Co-Authored-by: Iain Sandoe <iain@sandoe.co.uk>