[Bug c++/113360] New: Truncated constexpr error messages with -std=c++23/26

[Bug c++/113360] New: Truncated constexpr error messages with -std=c++23/26

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

            Bug ID: 113360
           Summary: Truncated constexpr error messages with -std=c++23/26
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Test by Paul Keir:

constexpr bool init_list()
{
  int total{};
  for (int x : {1, 2, 3})
    total += x;
  return total == 6;
}

static_assert(init_list(), "");

Using -std=c++23, the error message is:

<source>: In function 'constexpr bool init_list()':
<source>:6:24: error: deducing from brace-enclosed initializer list requires
'#include <initializer_list>'
  +++ |+#include <initializer_list>
    1 | // #include <initializer_list>
......
    6 |   for (int x : {1, 2, 3})
      |                        ^
<source>: At global scope:
<source>:11:24: error: non-constant condition for static assertion
   11 | static_assert(init_list(), "");
      |               ~~~~~~~~~^~
<source>:11:24: error: 'constexpr bool init_list()' called in a constant
expression
<source>:3:16: note: 'constexpr bool init_list()' is not usable as a
'constexpr' function because:
    3 | constexpr bool init_list()
      |                ^~~~~~~~~

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.3
            Summary|Truncated constexpr error   |[13/14 Regression]
                   |messages with -std=c++23/26 |Truncated constexpr error
                   |                            |messages with -std=c++23/26

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-01-12
             Status|UNCONFIRMED                 |NEW

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The "deducing from brace-enclosed initializer" error is emitted during parsing
from listify.  The body of init_list is not potentially-constant (due to
erroneous FOR_COND), so we register_constexpr_fundef it with result ==
error_mark_node.

require_rvalue_constant_expression in finish_static_assert thinks init_list is
potentially-constant, so we call cxx_constant_value.  new_call.fundef->result
is error_mark_node so we don't bother evaluating the body.  We call
explain_invalid_constexpr_fn but that has nothing to say, so we end up with a
missing reason.
But we already said what went wrong, so we shouldn't repeat it here anyway.

The body is:

{
  int total = 0;

  <<cleanup_point   int total = 0;>>;
  {
    int x = <<< error >>>;
    <<< error >>> __for_range;
    <<< error >>> __for_begin;
    <<< error >>> __for_end;

    <<< Unknown tree: for_stmt

      <<< error >>>
      <<< error >>>
      <<cleanup_point       int x = <<< error >>>;>>;
      <<cleanup_point <<< Unknown tree: expr_stmt
        (void) (total = total + VIEW_CONVERT_EXPR<int>(x)) >>>>>;
       >>>;
  }
  return <retval> = VIEW_CONVERT_EXPR<int>(total) == 6;
}

Can be reproduced in C++20 with -Wno-invalid-constexpr.

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Idea: use cp_function_chain->invalid_constexpr to not to attempt to
explain_invalid_constexpr_fn.

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

Jason Merrill <jason at gcc dot gnu.org> changed:

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

[Bug c++/113360] [13/14 Regression] Truncated constexpr error messages with -std=c++23/26

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113360

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |law at gcc dot gnu.org