[Bug c++/111703] [11/12/13 Regression] [C++20]Compiler fails when using generic lambda in specific situation since r11-550-gf65a3299a521a4

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111703

--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:6a31302a6c8bec9af3c93f736c2b1a76e9a530e2

commit r12-10016-g6a31302a6c8bec9af3c93f736c2b1a76e9a530e2
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Nov 15 12:03:16 2023 -0500

    c++: constantness of local var in constexpr fn [PR111703, PR112269]

    potential_constant_expression was incorrectly treating most local
    variables from a constexpr function as constant because it wasn't
    considering the 'now' parameter.  This patch fixes this by relaxing
    its var_in_maybe_constexpr_fn checks accordingly, which turns out to
    partially fix two recently reported regressions:

    PR111703 is a regression caused by r11-550-gf65a3299a521a4 for restricting
    constexpr evaluation during warning-dependent folding.  The mechanism is
    intended to restrict only constant evaluation of the instantiated
    non-dependent expression, but it also ends up restricting constant
    evaluation occurring during instantiation of the expression, in particular
    when instantiating the converted argument 'x' (a VIEW_CONVERT_EXPR) into
    a copy constructor call.  This seems like a flaw in the mechanism, though
    I don't know if we want to fix the mechanism or get rid of it completely
    since the original testcases which motivated the mechanism are fixed more
    simply by r13-1225-gb00b95198e6720.  In any case, this patch partially
    fixes this by making us correctly treat 'x' as non-constant which prevents
    the problematic warning-dependent folding from occurring at all.

    PR112269 is caused by r14-4796-g3e3d73ed5e85e7 for merging tsubst_copy
    into tsubst_copy_and_build.  tsubst_copy used to exit early when 'args'
    was empty, behavior which that commit deliberately didn't preserve.
    This early exit masked the fact that COMPLEX_EXPR wasn't handled by
    tsubst at all, and is a tree code that apparently we could see during
    warning-dependent folding on some targets.  A complete fix is to add
    handling for this tree code in tsubst_expr, but this patch should fix
    the reported testsuite failures since the COMPLEX_EXPRs that crop up
    in <complex> are considered non-constant expressions after this patch.

            PR c++/111703
            PR c++/112269

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case VAR_DECL>:
            Only consider var_in_maybe_constexpr_fn if 'now' is false.
            <case INDIRECT_REF>: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-fn8.C: New test.

    (cherry picked from commit 6665a8572c8f24bd55c6081c91f461442c94dcfb)