[Bug c++/98388] New: Throwing move-only parameter results in hard error in SFINAE context

[Bug c++/98388] New: Throwing move-only parameter results in hard error in SFINAE context

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

            Bug ID: 98388
           Summary: Throwing move-only parameter results in hard error in
                    SFINAE context
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
  Target Milestone: ---

https://godbolt.org/z/xn8fYz

====
struct moveonly {
    moveonly() = default;
    moveonly(moveonly&&) = default;
};

template<class T>
constexpr auto is_throwable(T t) -> decltype(throw t, true) {
    return true;
}
template<class T>
constexpr bool is_throwable(...) { return false; }

constexpr bool b = is_throwable<moveonly>(moveonly{});

====
<source>: In substitution of 'template<class T> constexpr decltype
((<throw-expression>, true)) is_throwable(T) [with T = moveonly]':
<source>:13:43:   required from here
<source>:7:46: error: use of deleted function 'constexpr
moveonly::moveonly(const moveonly&)'
    7 | constexpr auto is_throwable(T t) -> decltype(throw t, true) {
      |                                              ^~~~~~~
<source>:1:8: note: 'constexpr moveonly::moveonly(const moveonly&)' is
implicitly declared as deleted because 'moveonly' declares a move constructor
or move assignment operator
    1 | struct moveonly {
      |        ^~~~~~~~
<source>:7:52: note:   in thrown expression
    7 | constexpr auto is_throwable(T t) -> decltype(throw t, true) {
      |                                                    ^
====

I'm not sure what the value of `b` should be, but I believe that it should not
be a hard error. (Clang and MSVC initialize `b` to `true`.)

[Bug c++/98388] Throwing move-only parameter results in hard error in SFINAE context

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-02-04
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug c++/98388] Throwing move-only parameter results in hard error in SFINAE context

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
                 CC|                            |mpolacek at gcc dot gnu.org

[Bug c++/98388] Throwing move-only parameter results in hard error in SFINAE context

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

--- Comment #1 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:3a3e0f1b46a3ad71ebeedc419393e3a36f1ce6db

commit r14-8903-g3a3e0f1b46a3ad71ebeedc419393e3a36f1ce6db
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Feb 6 15:35:16 2024 -0500

    c++: make build_throw SFINAE-friendly [PR98388]

    Here the problem is that we give hard errors while substituting
    template parameters during overload resolution of is_throwable
    which has an invalid throw in decltype.

    The backtrace shows that fn_type_unification -> instantiate_template
    -> tsubst* passes complain=0 as expected, but build_throw doesn't
    have a complain parameter.  So let's add one.  Also remove a redundant
    local variable which I should have removed in my P2266 patch.

    There's still one problem for which I opened
<https://gcc.gnu.org/PR113853>.
    We need to patch up treat_lvalue_as_rvalue_p and remove the dg-bogus.

    Thanks to Patrick for notifying me of this PR.  This doesn't fully fix
    113789; there I think I'll have to figure our why a candidate wasn't
    discarded from the overload set.

            PR c++/98388

    gcc/cp/ChangeLog:

            * coroutines.cc (coro_rewrite_function_body): Pass
tf_warning_or_error
            to build_throw.
            (morph_fn_to_coro): Likewise.
            * cp-tree.h (build_throw): Adjust.
            * except.cc (expand_end_catch_block): Pass tf_warning_or_error to
            build_throw.
            (build_throw): Add a tsubst_flags_t parameter.  Use it.  Remove
            redundant variable.  Guard an inform call.
            * parser.cc (cp_parser_throw_expression): Pass tf_warning_or_error
            to build_throw.
            * pt.cc (tsubst_expr) <case THROW_EXPR>: Pass complain to
build_throw.

    libcc1/ChangeLog:

            * libcp1plugin.cc (plugin_build_unary_expr): Pass tf_error to
            build_throw.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/sfinae69.C: New test.

[Bug c++/98388] Throwing move-only parameter results in hard error in SFINAE context

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=113853

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mostly fixed, but bug 113853 remains.

[Bug c++/98388] Throwing move-only parameter results in hard error in SFINAE context

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98388

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed now.