Proposed patch here: https://gcc.gnu.org/pipermail/gcc-patches/2024-March/647120.html

The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:79420dd344145819677b3f975bb305a778fcaf91

commit r15-84-g79420dd344145819677b3f975bb305a778fcaf91
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Mon Mar 4 23:58:30 2024 +1100

    c++: Implement P2615 'Meaningful Exports' [PR107688]
    
    This clarifies which kinds of declarations may and may not be exported
    in various contexts. The patch additionally fixes up some small issues
    that were clarified by the paper.
    
    Most of the changes are with regards to export-declarations, which are
    applied for all standards modes that we support '-fmodules-ts' for.
    However there are also a couple of changes made to linkage specifiers
    ('extern "C"'); I've applied these as since C++20, to line up with when
    modules were actually introduced.
    
            PR c++/107688
    
    gcc/cp/ChangeLog:
    
            * name-lookup.cc (push_namespace): Error when exporting
            namespace with internal linkage.
            * parser.h (struct cp_parser): Add new flag
            'in_unbraced_export_declaration_p'.
            * parser.cc (cp_debug_parser): Print the new flag.
            (cp_parser_new): Initialise the new flag.
            (cp_parser_module_export): Set the new flag.
            (cp_parser_class_specifier): Clear and restore the new flag.
            (cp_parser_import_declaration): Imports can now appear directly
            in a linkage specification.
            (cp_parser_declaration): Categorise declarations as "name" or
            "special"; error on the later in contexts where the former is
            required.
            (cp_parser_class_head): Error when exporting a partial
            specialisation.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/contracts-1_a.C: Avoid now-illegal syntax.
            * g++.dg/modules/contracts-2_a.C: Likewise.
            * g++.dg/modules/contracts-3_a.C: Likewise.
            * g++.dg/modules/contracts-4_a.C: Likewise.
            * g++.dg/modules/lang-1_c.C: Clarify now-legal syntax.
            * g++.dg/modules/pr101582-1.C: Remove now-legal XFAILS.
            * g++.dg/template/crash71.C: Update error messages.
            * g++.dg/cpp2a/linkage-spec1.C: New test.
            * g++.dg/modules/export-3.C: New test.
            * g++.dg/modules/export-4_a.C: New test.
            * g++.dg/modules/export-4_b.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

Implemented for GCC 15.