[Bug c++/104507] [10/11 Regression] internal compiler error: unexpected expression ‘(int)(__ret)’ of kind cast_expr

["junk at sigpwr dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104507

--- Comment #2 from Daniel Nelson <junk at sigpwr dot com> ---
Had a few moments to do some testing/bisecting, and found the commit that fixed
this in mainline.

commit 9927ecbb42d5be48fa933adc26f8601fab5007ca
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 28 10:05:14 2021 -0400

    c++: quadratic constexpr behavior for left-assoc logical exprs [PR102780]

    In the testcase below the two left fold expressions each expand into a
    constant logical expression with 1024 terms, for which potential_const_expr
    takes more than a minute to return true.  This happens because p_c_e_1
    performs trial evaluation of the first operand of a &&/|| in order to
    determine whether to consider the potentiality of the second operand.
    And because the expanded expression is left-associated, this trial
    evaluation causes p_c_e_1 to be quadratic in the number of terms of the
    expression.

    This patch fixes this quadratic behavior by making p_c_e_1 preemptively
    compute potentiality of the second operand of a &&/||, and perform trial
    evaluation of the first operand only if the second operand isn't
    potentially constant.  We must be careful to avoid emitting bogus
    diagnostics during the preemptive computation; to that end, we perform
    this shortcut only when tf_error is cleared, and when tf_error is set we
    now first check potentiality of the whole expression quietly and replay
    the check noisily for diagnostics.

    Apart from fixing the quadraticness for left-associated logical exprs,
    this change also reduces compile time for the libstdc++ testcase
    20_util/variant/87619.cc by about 15% even though our <variant> uses
    right folds instead of left folds.  Likewise for the testcase in the PR,
    for which compile time is reduced by 30%.  The reason for these speedups
    is that p_c_e_1 no longer performs expensive trial evaluation of each term
    of large constant logical expressions when determining their potentiality.

            PR c++/102780

    gcc/cp/ChangeLog:

            * constexpr.c (potential_constant_expression_1) <case
TRUTH_*_EXPR>:
            When tf_error isn't set, preemptively check potentiality of the
            second operand before performing trial evaluation of the first
            operand.
            (potential_constant_expression_1): When tf_error is set, first
check
            potentiality quietly and return true if successful, otherwise
            proceed noisily to give errors.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/fold13.C: New test.

 gcc/cp/constexpr.c                  | 26 +++++++++++++++++++++-----
 gcc/testsuite/g++.dg/cpp1z/fold13.C | 29 +++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/fold13.C