[Bug c++/99174] [modules] ICE with recursive inclusion of header-unit

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99174

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathan Sidwell <nathan@gcc.gnu.org>:

https://gcc.gnu.org/g:afed55036b65526be006d55f003f670ebeadb223

commit r11-7324-gafed55036b65526be006d55f003f670ebeadb223
Author: Nathan Sidwell <nathan@acm.org>
Date:   Mon Feb 22 07:43:56 2021 -0800

    c++: Recursing header imports and other duplicates [PR 99174]

    The fix for 98741 introduced two issues.  (a) recursive header units
    iced because we tried to read the preprocessor state after having
    failed to read the config.  (b) we could have duplicate imports of
    named modules in our pending queue, and that would lead to duplicate
    requests for pathnames, which coupled with the use of a null-pathname
    to indicate we'd asked could lead to desynchronization with the module
    mapper.  Fixed by adding a 'visited' flag to module state, so we ask
    exactly once.

            PR c++/99174
            gcc/cp
            * module.cc (struct module_state): Add visited_p flag.
            (name_pending_imports): Use it to avoid duplicate requests.
            (preprocess_module): Don't read preprocessor state if we failed to
            load a module's config.
            gcc/testsuite/
            * g++.dg/modules/pr99174-1_a.C: New.
            * g++.dg/modules/pr99174-1_b.C: New.
            * g++.dg/modules/pr99174-1_c.C: New.
            * g++.dg/modules/pr99174.H: New.