[Bug libstdc++/86590] Codegen is poor when passing std::string by value with _GLIBCXX_EXTERN_TEMPLATE undefined

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86590

--- Comment #25 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So, apparently the first bullet is wherever the constant-expression
non-terminal appears in the grammar rather than that e is a constant
expression.

So the above patch really needs to be:
+      if (ctx->quiet && !ctx->maybe_constant_init)
+       {
+         *non_constant_p = true;
+         return t;
+       }
where ctx->maybe_constant_init flag would be set in some special variant of
maybe_constant_init_1.  store_init_value has:
  /* In C++11 constant expression is a semantic, not syntactic, property.
     In C++98, make sure that what we thought was a constant expression at
     template definition time is still constant and otherwise perform this
     as optimization, e.g. to fold SIZEOF_EXPRs in the initializer.  */
  if (decl_maybe_constant_var_p (decl) || TREE_STATIC (decl))
    {
      bool const_init;
      tree oldval = value;
      value = fold_non_dependent_expr (value);
      if (DECL_DECLARED_CONSTEXPR_P (decl)
          || (DECL_IN_AGGR_P (decl)
              && DECL_INITIALIZED_IN_CLASS_P (decl)
              && !DECL_VAR_DECLARED_INLINE_P (decl)))
        {
          /* Diagnose a non-constant initializer for constexpr variable or
             non-inline in-class-initialized static data member.  */
          if (!require_constant_expression (value))
            value = error_mark_node;
          else
            value = cxx_constant_init (value, decl);
        }
      else
        value = maybe_constant_init (value, decl);
and the cxx_constant_init call there already sets ctx->quiet to false, so the
maybe_constant_init call here is probably the one that would need to be
changed.
There are tons of other maybe_constant_init calls in the C++ FE though, and it
is unclear if any other would need changes (wonder e.g. about aggregate
initializers or initializer-list etc. that contain many separate expressions;
if we need is_constant_evaluated to be true sometimes in there, it would need
to check if they are all constant expressions).