[Bug c++/123889] [13/14/15 Regression] C++11 diagnostic about non-constexpr functions is useless

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123889

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-15 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:627283aa16cd2825763d4f9919343cf823d2feac

commit r15-10900-g627283aa16cd2825763d4f9919343cf823d2feac
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Feb 9 09:06:47 2026 +0100

    c++: Fix up diagnostics of wrong constexpr bodies in C++11 in templates
[PR123889]

    We emit weird diagnostics on the following testcase in C++11.
    If it is not a template, maybe_save_constexpr_fundef calls first
      if (!is_valid_constexpr_fn (fun, complain))
        return;
    (which doesn't fail) and then
      tree massaged = massage_constexpr_body (fun, DECL_SAVED_TREE (fun));
      if (massaged == NULL_TREE || massaged == error_mark_node)
        {
          if (!DECL_CONSTRUCTOR_P (fun) && complain)
            error ("body of %<constexpr%> function %qD not a return-statement",
                   fun);
          return;
        }
    which diagnoses it and if even that would succeed, go on with
      bool potential = potential_rvalue_constant_expression (massaged);
      if (!potential && complain)
        require_potential_rvalue_constant_expression_fncheck (massaged);
    In templates, maybe_save_constexpr_fundef returns early:
      if (processing_template_decl
          || cp_function_chain->invalid_constexpr
          || (DECL_CLONED_FUNCTION_P (fun) && !DECL_DELETING_DESTRUCTOR_P
(fun)))
        return;
    and then it is called again during instantiation.
    But in that case DECL_GENERATED_P (fun) is true and so we silently return
    on errors without diagnosing them:
      bool complain = !DECL_GENERATED_P (fun) && !implicit;
    Now, when we actually try to constexpr evaluate those (if at all), we
    emit an error and then
    'constexpr ...' is not usable as a 'constexpr' function because:
    message and then explain_invalid_constexpr_fn tries to diagnose
    the errors by calling is_valid_constexpr_fn (fun, true) and
    require_potential_rvalue_constant_expression (massaged).  So it diagnoses
    those 2 cases, but misses the one where massaged was NULL or
error_mark_node
    for a non-constructor, so after the because: there is no reason emitted.

    The following patch diagnoses even that.

    2026-02-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/123889
            * constexpr.cc (explain_invalid_constexpr_fn): Diagnose
            NULL or error_mark_node massaged on non-constructor.

            * g++.dg/cpp0x/constexpr-123889.C: New test.

    (cherry picked from commit 4e13b78bf0e28120d4b1595313c42a1de83fc3ed)