[Bug c++/119097] Modules treats non-ODR usages of constexpr values as exposure

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119097

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:c388d56a889469cf05a1512403e3267af7bb62be

commit r16-6431-gc388d56a889469cf05a1512403e3267af7bb62be
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Nov 7 23:24:17 2025 +1100

    c++: Fold non-ODR usages of potentially constant values early [PR120005]

    [basic.link] p14.4 says that a declaration naming a TU-local entity is
    an exposure, ignoring "any reference to a non-volatile const object or
    reference with internal or no linkage initialized with a constant
    expression that is not an odr-use".

    To implement this, we cannot stream these entities but must fold them
    into their underlying values beforehand.  This was already done to a
    degree by cp_fold, but it didn't handle all cases, and notably was not
    performed on the saved body of a constexpr function, which would then
    cause errors during modules streaming.

    This patch implements this by supplementing cp_fold with additional
    rules to fold non-ODR usages of contants.  We need to do this as an
    additional walk before saving the constexpr function definition, so we
    also disable as much other folding during this walk as possible to
    prevent removing any information that the constexpr interpreter
    requires to function correctly.

    With this we still will error on uses in templates.  In general it's
    impossible to tell within an uninstantiated template body whether a
    reference is an ODR-use in the face of dependent expressions, so we
    don't attempt to do anything for this case.

            PR c++/119097
            PR c++/120005

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1): Fall back to
            location from parent expression if needed.
            * cp-gimplify.cc (enum fold_flags): Add ff_only_non_odr.
            (cp_fold_data::cp_fold_data): Assert invariant for flags.
            (cp_fold_omp_clause_refs_r): New function.
            (cp_fold_r): Specially handle OMP_CLAUSE_DECL.
            (cp_fold_function_non_odr_use): New function.
            (cp_fold_non_odr_use_1): New function.
            (cp_fold_maybe_rvalue): Fold non-ODR uses when requested.
            (cp_fold_non_odr_use): New function.
            (fold_caches): Increase number of caches.
            (get_fold_cache): Use a new cache for non-ODR use walks.
            (cp_fold): Skip most folding for non-ODR use walks; always
            fold constant-initialized references; remove dead code to
            fold __builtin_source_location.
            * cp-tree.h (cp_fold_function_non_odr_use): Declare.
            (cp_fold_non_odr_use): Declare.
            * decl.cc (finish_function): Fold non-ODR uses before saving
            constexpr fundef.  Invoke PLUGIN_PRE_GENERICIZE before this
            folding.
            * ptree.cc (cxx_print_xnode): Handle TU_LOCAL_ENTITY.
            * tree.cc (bot_manip): Propagate TREE_CONSTANT.
            * typeck2.cc (digest_nsdmi_init): Fold non-ODR uses in NSDMIs.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-cast.C: Adjust diagnostics.
            * g++.dg/warn/overflow-warn-1.C: Fix diagnostic checks.
            * g++.dg/warn/overflow-warn-3.C: Likewise.
            * g++.dg/warn/overflow-warn-4.C: Likewise.
            * g++.dg/modules/internal-8_a.C: Remove xfails, supplement with
            additional testcases.
            * g++.dg/modules/internal-8_b.C: New test.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>