[Bug c++/96533] New: ICE with -Wunused-parameter when using three-way comparison

[Bug c++/96533] New: ICE with -Wunused-parameter when using three-way comparison

[natattak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

            Bug ID: 96533
           Summary: ICE with -Wunused-parameter when using three-way
                    comparison
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: natattak at gmail dot com
  Target Milestone: ---

Minimal example:

  #include <compare>
  #include <functional>

  struct S {
    // making it a member fixes the issue
    friend auto operator<=>(S, S) = default;
  };

  // removing 'rhs' fixes the issue
  template <typename Lhs, typename Rhs>
  auto operator<(Lhs&&, Rhs&& rhs) {
    return true;
  }

  int main() {
    // using operator< directly fixes the issue
    //return S{} < S{};
    return std::less{}(S{}, S{});
  }

Compiled with `g++ -std=c++20 -Wunused-parameter`. Output:

  '
  Internal compiler error: Error reporting routines re-entered.
  Please submit a full bug report,
  with preprocessed source if appropriate.
  See <https://gcc.gnu.org/bugs/> for instructions.

First noticed on g++ 10.1, tested on compiler explorer with current trunk:
https://godbolt.org/z/85j8TW

[Bug c++/96533] ICE with -Wunused-parameter when using three-way comparison

[natattak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

--- Comment #1 from Nathaniel Shead <natattak at gmail dot com> ---
On further investigation, I suspect this is more general than just
`-Wunused-parameter`; the error looks to be caused by any error occurring
within `operator<`. For example,

  #include <compare>
  #include <functional>

  struct S {};
  auto operator<=>(S, S) = default;

  template <typename Lhs, typename Rhs>
  auto operator<(Lhs&&, Rhs&& rhs) {
    return rhs.nonexistent;
  }

  int main() {
    return std::less{}(S{}, S{});
  }

produces the same issue.

I also discovered that the issue is only when the templated `operator<` has
deduced type.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload

[natattak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

Nathaniel Shead <natattak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ICE with -Wunused-parameter |ICE with three-way
                   |when using three-way        |comparison when error
                   |comparison                  |occurs in templated
                   |                            |operator< overload

--- Comment #2 from Nathaniel Shead <natattak at gmail dot com> ---
(Correction with my previous comment, apologies; should be 

  struct S {};
  auto operator<=>(S, S) { return std::strong_ordering::equal; }

— see https://godbolt.org/z/aPxv8d, or otherwise a friend as in my first post.)

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reducing ....

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 52014
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52014&action=edit
Reduced as I can get it right now

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|12.0                        |

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I can only figure out that it has something to do with __ptr_cmp template
class.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #52014|0                           |1
        is obsolete|                            |

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 52015
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52015&action=edit
A little smaller

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #52015|0                           |1
        is obsolete|                            |

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 52016
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52016&action=edit
A little more

The main thing is __not_overloaded and the other operator< really.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
0xccadfb do_warn_unused_parameter(tree_node*)
        /home/apinski/src/upstream-gcc/gcc/gcc/c-family/c-warn.c:2414
0xa74de1 finish_function(bool)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/decl.c:17816
0xba9b36 instantiate_body
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/pt.c:26176
0xbaad6a instantiate_decl(tree_node*, bool, bool)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/pt.c:26450
0xa8586a maybe_instantiate_decl(tree_node*)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/decl2.c:5594
0xa86aac maybe_instantiate_decl(tree_node*)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/decl2.c:5830
0xa86aac mark_used(tree_node*, int)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/decl2.c:5856
0x9ce2ce build_over_call
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/call.c:9819
0x9d4f59 build_new_op(op_location_t const&, tree_code, int, tree_node*,
tree_node*, tree_node*, tree_node**, int)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/call.c:6685
0x9d5296 build_new_op(op_location_t const&, tree_code, int, tree_node*,
tree_node*, tree_node*, tree_node**, int)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/call.c:6768
0xc1ead3 build_x_binary_op(op_location_t const&, tree_code, tree_node*,
tree_code, tree_node*, tree_code, tree_node**, int)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/typeck.c:4480
0xb95898 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool,
bool)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/pt.c:20074
0xba60ce tsubst(tree_node*, tree_node*, int, tree_node*)
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/pt.c:16181
0xa97631 dump_template_bindings
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/error.c:486
0xa90a8a dump_function_decl
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/error.c:1806
0xa98dc4 decl_to_string
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/error.c:3226
0xa98dc4 cp_printer
        /home/apinski/src/upstream-gcc/gcc/gcc/cp/error.c:4397
0x2118a6c pp_format(pretty_printer*, text_info*)
        /home/apinski/src/upstream-gcc/gcc/gcc/pretty-print.c:1475
0x211ac00 pp_format_verbatim(pretty_printer*, text_info*)
        /home/apinski/src/upstream-gcc/gcc/gcc/pretty-print.c:1536
0x211ac00 pp_verbatim(pretty_printer*, char const*, ...)
        /home/apinski/src/upstream-gcc/gcc/gcc/pretty-print.c:1790
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-12-16
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.  do_warn_unused_parameter ends up calling itself, oops.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |12.1.0, 13.1.0
      Known to fail|                            |10.5.0, 11.4.0
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #10 from Patrick Palka <ppalka at gcc dot gnu.org> ---
This seems fixed for GCC 12+ by r12-6022-gbb2a7f80a98de3.

[Bug c++/96533] ICE with three-way comparison when error occurs in templated operator< overload and -Wunused-parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96533

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
   Target Milestone|---                         |12.0
         Resolution|---                         |FIXED

--- Comment #11 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thus fixed.  Adding a testcase for this issue doesn't seem super worthwhile.