[Bug c++/113853] New: implicit move in throw in trailing return type

[Bug c++/113853] New: implicit move in throw in trailing return type

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113853

            Bug ID: 113853
           Summary: implicit move in throw in trailing return type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Found while working on bug 98388.  This should compile fine in all dialects:

```
// { dg-do compile { target c++11 } }

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{});
#if __cplusplus >= 202002L
static_assert (b, "move from the function parameter");
#else
static_assert (!b, "no move from the function parameter");
#endif
```

but it doesn't; see
<https://gcc.gnu.org/pipermail/gcc-patches/2024-February/645275.html>.

[Bug c++/113853] implicit move in throw in trailing return type

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113853

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
   Last reconfirmed|                            |2024-02-09
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid

[Bug c++/113853] implicit move in throw in trailing return type

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113853

Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arthur.j.odwyer at gmail dot com

--- Comment #1 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
(Sorry, not sure where's the best place to put this comment. Maybe nowhere. ;))
You probably know this already, but just in case anyone's not on the same page
yet:

https://eel.is/c++draft/except.throw#5
> When the thrown object is a class object, the constructor selected for the copy-initialization *as well as the constructor selected for a copy-initialization considering the thrown object as an lvalue* shall be non-deleted and accessible, even if the copy/move operation is elided.

That paragraph is lurking in the background every time we talk about GCC's
handling of move-only exception objects. However, my impression is that the
paragraph is ignorable from the vendor's point of view:

- Even on paper, this "shall" is talking only about the runtime behavior "when
an exception _is_ thrown," so it doesn't change anything about the
well-formedness of _unevaluated_ throw-expressions.

- GCC+Clang _probably could_ conformingly decide that it should be ill-formed
to `throw x` when `x` is move-only; but that would break a lot of users, so
pragmatically they can't do that. GCC+Clang need a consistent theory of what it
means to throw a move-only type, and the theory can't possibly be "it's
ill-formed" (because that would break users), so the theory must agree with
Marek's test case: `throw t` ought to be well-formed in C++20-and-later (by
wording introduced to C++20 in P1155 a.k.a. P1825).

IOW, this bug and test case are *not* invalidated by [except.throw]/5. That's
all. :)

[Bug c++/113853] implicit move in throw in trailing return type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113853

--- Comment #2 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:254ff3d0e34835b4de93d5e5763a7366dc7d989d

commit r14-9034-g254ff3d0e34835b4de93d5e5763a7366dc7d989d
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Feb 14 17:53:52 2024 -0500

    c++: implicit move with throw [PR113853]

    Here we have

      template<class T>
      auto is_throwable(T t) -> decltype(throw t, true) { ... }

    where we didn't properly mark 't' as IMPLICIT_RVALUE_P, which caused
    the wrong overload to have been chosen.  Jason figured out it's because
    we don't correctly implement [expr.prim.id.unqual]#4.2, which post-P2266
    says that an id-expression is move-eligible if

    "the id-expression (possibly parenthesized) is the operand of
    a throw-expression, and names an implicitly movable entity that belongs
    to a scope that does not contain the compound-statement of the innermost
    lambda-expression, try-block, or function-try-block (if any) whose
    compound-statement or ctor-initializer contains the throw-expression."

    I worked out that it's trying to say that given

      struct X {
        X();
        X(const X&);
        X(X&&) = delete;
      };

    the following should fail: the scope of the throw is an sk_try, and it's
    also x's scope S, and S "does not contain the compound-statement of the
    *try-block" so x is move-eligible, so we move, so we fail.

      void f ()
      try {
        X x;
        throw x;  // use of deleted function
      } catch (...) {
      }

    Whereas here:

      void g (X x)
      try {
        throw x;
      } catch (...) {
      }

    the throw is again in an sk_try, but x's scope is an sk_function_parms
    which *does* contain the {} of the *try-block, so x is not move-eligible,
    so we don't move, so we use X(const X&), and the code is fine.

    The current code also doesn't seem to handle

      void h (X x) {
        void z (decltype(throw x, true));
      }

    where there's no enclosing lambda or sk_try so we should move.

    I'm not doing anything about lambdas because we shouldn't reach the
    code at the end of the function: the DECL_HAS_VALUE_EXPR_P check
    shouldn't let us go further.

            PR c++/113789
            PR c++/113853

    gcc/cp/ChangeLog:

            * typeck.cc (treat_lvalue_as_rvalue_p): Update code to better
            reflect [expr.prim.id.unqual]#4.2.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/sfinae69.C: Remove dg-bogus.
            * g++.dg/cpp0x/sfinae70.C: New test.
            * g++.dg/cpp0x/sfinae71.C: New test.
            * g++.dg/cpp0x/sfinae72.C: New test.
            * g++.dg/cpp2a/implicit-move4.C: New test.

[Bug c++/113853] implicit move in throw in trailing return type

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113853

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

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

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed in GCC 14.