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

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.