[Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575

--- Comment #3 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:46fdced6a9f936ae4d5b42347d7d87f69875683a

commit r11-4230-g46fdced6a9f936ae4d5b42347d7d87f69875683a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 22 07:33:58 2020 -0400

    c++: constexpr evaluation and bare EMPTY_CLASS_EXPR [PR96575]

    In the testcase below, folding of the initializer for 'ret' inside the
    instantiated f<lambda>::lambda ends up yielding an initializer for which
    potential_constant_expression returns false.  This causes finish_function
    to mark the lambda as non-constexpr, which ultimately causes us to reject
    'f(g)' as a call to a non-constexpr function.

    The initializer for 'ret' inside f<lambda>::lambda, prior to folding, is
    the CALL_EXPR

      <lambda(S)>::operator() (&cb, ({}, <<< Unknown tree: empty_class_expr
>>>;))

    where the second argument is a COMPOUND_EXPR whose second operand is an
    EMPTY_CLASS_EXPR that was formed by build_class_a.  cp_fully_fold_init
    is able to only partially fold this initializer: it gets rid of the
    side-effectless COMPOUND_EXPR to obtain

      <lambda(S)>::operator() (&cb, <<< Unknown tree: empty_class_expr >>>)

    as the final initializer for 'ret'.  This initializer no longer satifies
    potential_constant_expression due to the bare EMPTY_CLASS_EXPR which is
    not wrapped in a COMPOUND_EXPR.

    (cp_fully_fold_init first tries maybe_constant_value on the original
    CALL_EXPR, but constexpr evaluation punts upon seeing
    __builtin_is_constant_evaluated, since manifestly_const_eval is false.)

    To fix this, it seems we could either make cp_fold preserve the
    COMPOUND_EXPR trees produced by build_call_a, or we could improve
    the constexpr machinery to treat EMPTY_CLASS_EXPR trees as first-class
    citizens.  Assuming it's safe to continue folding away these
    COMPOUND_EXPRs, the second approach seems cleaner, so this patch
    implements the second approach.

    gcc/cp/ChangeLog:

            PR c++/96575
            * constexpr.c (cxx_eval_constant_expression)
            <case EMPTY_CLASS_EXPR>: Lower it to a CONSTRUCTOR.
            (potential_constant_expression_1) <case COMPOUND_EXPR>: Remove
            now-redundant handling of COMPOUND_EXPR with EMPTY_CLASS_EXPR
            second operand.
            <case EMPTY_CLASS_EXPR>: Return true instead of false.

    gcc/testsuite/ChangeLog:

            PR c++/96575
            * g++.dg/cpp1z/constexpr-96575.C: New test.