[Bug c++/113789] New: ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[Bug c++/113789] New: ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

            Bug ID: 113789
           Summary: ICE on P2266/C++23 `decltype(throw x)` where x is
                    move-eligible parameter
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

P2266 "Simpler Implicit Move", adopted for C++23, intends that a move-eligible
`x` should be treated as an xvalue in the context of `throw x`. This means that
`throw x` should be ill-formed (SFINAE-friendly) when `x` isn't
move-constructible.

(This might be complicated by the GCC+Clang extension that permits move-only
types to be thrown even though the paper Standard technically prohibits that.)

That is, the test program in this blog post is expected to pass after P2266:
https://quuxplusone.github.io/blog/2021/03/18/sfinae-on-throw-x/#here-is-a-program-that-uses-sfin
Clang passes; GCC fails; MSVC and EDG don't seem to implement P2266 `throw`
yet.

But the real problem for GCC is that if you then use `throw x` inside the
function body after using `decltype(throw x)` in the function's return type,
the compiler ICEs!


// https://godbolt.org/z/YG16hE7zT
struct AutoPtr {
    AutoPtr() = default;
    AutoPtr(AutoPtr&) {}
};
template<class T> auto f(T p, int) -> decltype(throw p, 1) { throw p; }
template<class T> int f(T p, long) { return 2; }
int main() {
    return f(AutoPtr(), 42);
}


The expected behavior (and Clang's behavior) is to call `f(T, long)`. GCC 12
generates a call to `f(T, int)`. GCC 13 crashes with this output (including
that leading single-quote):
'
internal compiler error: error reporting routines re-entered.
0x2648345 diagnostic_context::report_diagnostic(diagnostic_info*)
        ???:0
0x26494c5 error_at(unsigned int, char const*, ...)
        ???:0
0xa7d834 build_new_method_call(tree_node*, tree_node*, vec<tree_node*, va_gc,
vl_embed>**, tree_node*, int, tree_node**, int)
        ???:0
0xa7eb5b build_special_member_call(tree_node*, tree_node*, vec<tree_node*,
va_gc, vl_embed>**, tree_node*, int, int)
        ???:0
0xb685e4 build_throw(unsigned int, tree_node*)
        ???:0
0xc95e47 tsubst(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x267035c pp_format(pretty_printer*, text_info*, urlifier const*)
        ???:0
0x2673055 pp_verbatim(pretty_printer*, char const*, ...)
        ???:0
0x26481ca diagnostic_context::report_diagnostic(diagnostic_info*)
        ???:0
0x26494c5 error_at(unsigned int, char const*, ...)
        ???:0
0xa7d834 build_new_method_call(tree_node*, tree_node*, vec<tree_node*, va_gc,
vl_embed>**, tree_node*, int, tree_node**, int)
        ???:0
0xa7eb5b build_special_member_call(tree_node*, tree_node*, vec<tree_node*,
va_gc, vl_embed>**, tree_node*, int, int)
        ???:0
0xb685e4 build_throw(unsigned int, tree_node*)
        ???:0
0xc8b5c3 instantiate_decl(tree_node*, bool, bool)
        ???:0
0xcb56cb instantiate_pending_templates(int)
        ???:0
0xb54d39 c_parse_final_cleanups()
        ???:0
0xda8358 c_common_parse_file()
        ???:0

[Bug c++/113789] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ice-on-valid-code

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>GCC 12 generates a call to `f(T, int)`. GCC 13 crashes with this output

THis is because the paper was not implemented before GCC 13; see the linked bug
reports for that.

[Bug c++/113789] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-02-06
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r13-2905

commit 772d532e0ba1e4b22c2b7d576e14b34ee929c093
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Aug 31 17:37:59 2022 -0400

    c++: Implement C++23 P2266R1, Simpler implicit move [PR101165]

so mine.

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ICE on P2266/C++23          |[13/14 Regression] ICE on
                   |`decltype(throw x)` where x |P2266/C++23 `decltype(throw
                   |is move-eligible parameter  |x)` where x is
                   |                            |move-eligible parameter
   Target Milestone|---                         |13.3
           Priority|P3                          |P2

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I have a patch for the ICE (build_throw doesn't have a complain param so we
wind up with "error reporting routines re-entered").

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Um, that's not it.  Since

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

template<class T> int
f (T p)
{
  throw p;
}

void
g ()
{
  f (AutoPtr ());
}

is rejected in C++23, we probably should have rejected the decltype(throw p, 1)
candidate and never get to the throw in the body of the function f.

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
IOW, this should be accepted in C++23 but isn't (clang++ accepts in C++23):

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

template<class T> auto f(T p, int) -> decltype(throw p, 1) = delete;
template<class T> void f(T p, long);

void
g ()
{
  f (AutoPtr (), 42);
}

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #6 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
(In reply to Marek Polacek from comment #5)
> IOW, this should be accepted in C++23 but isn't (clang++ accepts in C++23): [...]

Correct, at least that's my intended interpretation. But the unexpected ICE was
more dramatic. :)
I'd be a tiny bit surprised if fixing `decltype(throw p)` actually closes the
only pathway to that ICE; but if it does, then awesome, I have no complaints.

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The ICE was fixed by r14-8903-g3a3e0f1b46a3ad.  But I'm not done here yet.

[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #8 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++/113789] [13 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
            Summary|[13/14 Regression] ICE on   |[13 Regression] ICE on
                   |P2266/C++23 `decltype(throw |P2266/C++23 `decltype(throw
                   |x)` where x is              |x)` where x is
                   |move-eligible parameter     |move-eligible parameter
         Resolution|---                         |FIXED

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fully fixed now.

[Bug c++/113789] [13 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #10 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
FWIW, I think I agree with your analysis. To reiterate what you already said
(and I think GCC already gets the following snippet correct): in

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

the `throw x` copies but the `return x` moves. That is, `throw x` treats `x` as
an lvalue because it could be used again later (in the function-catch-block),
but `return x` treats it as an rvalue because it can't[*] be used again later.

[* — except if you sneakily use a captured reference within a destructor, but
C++ implicit move doesn't care — has never cared — about such sneaky uses]