[Bug c++/108550] [10/11/12 Regression] the type 'const auto' of 'constexpr' variable is not literal

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108550

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:d6419c18056ae27741e961cd2dab9d120bfe746a

commit r12-9219-gd6419c18056ae27741e961cd2dab9d120bfe746a
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Mar 4 13:14:01 2023 -0500

    c++: variable template and targ deduction [PR108550]

    In this test, we get a bogus error because we failed to deduce the auto in
    constexpr auto is_pointer_v = is_pointer<Tp>::value;
    to bool.  Then ensure_literal_type_for_constexpr_object thinks the object
    isn't literal and an error is reported.

    This is another case of the interaction between tf_partial and 'auto',
    where the auto was not reduced so the deduction failed.  In more detail:
    we have

      Wrap1<int>()

    in the code and we need to perform OR -> fn_type_unification.  The targ
    list is incomplete, so we do
          tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
          fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain,
NULL_TREE);
    where TREE_TYPE (fn) is struct integral_constant <T402> (void).  Then
    we substitute the return type, which results in tsubsting
is_pointer_v<int>.
    is_pointer_v is a variable template with a placeholder type:

      template <class Tp>
      constexpr auto is_pointer_v = is_pointer<Tp>::value;

    so we find ourselves in lookup_and_finish_template_variable.  tf_partial is
    still set, so finish_template_variable -> instantiate_template -> tsubst
    won't reduce the level of auto.  But then we do mark_used which eventually
    calls do_auto_deduction which clears tf_partial, because we want to replace
    the auto now.  But we hadn't reduced auto's level so this fails.  And
    since we're not in an immediate context, we emit a hard error.

    I suppose that when we reach lookup_and_finish_template_variable it's
    probably time to clear tf_partial.  (I added an assert and our testsuite
    doesn't have a test whereby we get to lookup_and_finish_template_variable
    while tf_partial is still active.)

            PR c++/108550

    gcc/cp/ChangeLog:

            * pt.cc (lookup_and_finish_template_variable): Clear tf_partial.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ70.C: New test.
            * g++.dg/cpp1y/var-templ71.C: New test.
            * g++.dg/cpp1y/var-templ72.C: New test.