[Bug libstdc++/97600] [ranges] satisfaction value of range<basic_istream_view> affected by prior use of basic_istream_view::begin()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97600

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:9324f7a25c7161a813bfae6cc2d180784b165740

commit r11-5954-g9324f7a25c7161a813bfae6cc2d180784b165740
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Dec 11 14:37:09 2020 -0500

    c++: Avoid considering some conversion ops [PR97600]

    Patrick's earlier patch to check convertibility before constraints for
    conversion ops wasn't suitable because checking convertibility can also
lead
    to unwanted instantiations, but it occurs to me that there's a smaller
check
    we can do to avoid doing normal consideration of the conversion ops in this
    case: since we're in the middle of a user-defined conversion, we can
exclude
    from consideration any conversion ops that return a type that would need an
    additional user-defined conversion to reach the desired type: namely, a
type
    that differs in class-ness from the desired type.

    [temp.inst]/9 allows optimizations like this: "If the function selected by
    overload resolution can be determined without instantiating a class
template
    definition, it is unspecified whether that instantiation actually takes
    place."

    gcc/cp/ChangeLog:

            PR libstdc++/97600
            * call.c (build_user_type_conversion_1): Avoid considering
            conversion functions that return a clearly unsuitable type.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-conv3.C: New test.