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

[mpolacek at gcc dot gnu.org via Gcc-bugs]

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>.