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

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

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

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c8aaa9cca96207b5674048972c82a338ef81ce7e

commit r11-9843-gc8aaa9cca96207b5674048972c82a338ef81ce7e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Feb 16 12:41:35 2022 -0500

    c++: treat NON_DEPENDENT_EXPR as not potentially constant [PR104507]

    Here we're crashing from potential_constant_expression because it tries
    to perform trial evaluation of the first operand '(bool)__r' of the
    conjunction (which is overall wrapped in a NON_DEPENDENT_EXPR), but
    cxx_eval_constant_expression ICEs on unsupported trees (of which CAST_EXPR
    is one).  The sequence of events is:

      1. build_non_dependent_expr for the array subscript yields
         NON_DEPENDENT_EXPR<<<(bool)__r && __s>>> ? 1 : 2
      2. cp_build_array_ref calls fold_non_dependent_expr on this subscript
         (after this point, processing_template_decl is cleared)
      3. during which, the COND_EXPR case of tsubst_copy_and_build calls
         fold_non_dependent_expr on the first operand
      4. during which, we crash from p_c_e_1 because it attempts trial
         evaluation of the CAST_EXPR '(bool)__r'.

    Note that even if this crash didn't happen, fold_non_dependent_expr
    from cp_build_array_ref would still ultimately be one big no-op here
    since neither constexpr evaluation nor tsubst handle NON_DEPENDENT_EXPR.

    In light of this and of the observation that we should never see
    NON_DEPENDENT_EXPR in a context where a constant expression is needed
    (it's used primarily in the build_x_* family of functions), it seems
    futile for p_c_e_1 to ever return true for NON_DEPENDENT_EXPR.  And the
    otherwise inconsistent handling of NON_DEPENDENT_EXPR between p_c_e_1,
    cxx_evaluate_constexpr_expression and tsubst apparently leads to weird
    bugs such as this one.

            PR c++/104507

    gcc/cp/ChangeLog:

            * constexpr.c (potential_constant_expression_1)
            <case NON_DEPENDENT_EXPR>: Return false instead of recursing.
            Assert tf_error isn't set.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/non-dependent21.C: New test.

    (cherry picked from commit c19f317a78c0e4c1b51d0e5a8e4c0a3b985b7a8e)