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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104255] parsing function signature fails when it uses a
 function parameter outside of an unevaluated context
Date: Wed, 02 Feb 2022 15:13:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104255

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to qingzhe huang from comment #4)
> (In reply to Patrick Palka from comment #2)
>=20
> >=20
> >   error: use of parameter outside function body before =E2=80=98)=E2=80=
=99 token
> >=20
> > due to 'e' being used outside of an unevaluated context within the sign=
ature
> > of the function.
>=20
> Sorry for my being unable to grasp your meaning before. Now I can see your
> point that the "e" of A<f(e)> is from declaration of parameter of function
> "g". Now that we agree the value from parameter clause should not be used=
 in
> trailing return type, then it should also not be used in requires clause
> etc.=20
> (https://eel.is/c++draft/basic.scope.param#1.3)
> But this works:
>=20
> template<typename T>
> auto f(int n) requires (n>0);

That's because the constraint-expression of a requires clause is an unevalu=
ated
operand (wg21.link/temp.pre#9.sentence-4).  (Though I think this example is
strictly speaking ill-formed no diagnostic required because the constraint =
can
never be satisfied (wg21.link/temp.res.general#6.2).)

The check in question is in finish_id_expression_1:

      /* Also disallow uses of function parameters outside the function
         body, except inside an unevaluated context (i.e. decltype).  */
      if (TREE_CODE (decl) =3D=3D PARM_DECL
          && DECL_CONTEXT (decl) =3D=3D NULL_TREE
          && !cp_unevaluated_operand)
        {
          *error_msg =3D G_("use of parameter outside function body");
          return error_mark_node;
        }

>=20
> This is violating our assumption. So, I am not convinced that=20
> (https://eel.is/c++draft/basic.scope.param#note-1)
> "A function parameter cannot be used for its value within the
> parameter-declaration-clause" is really meaningful. Or I misunderstand it=
???

We should probably relax the above check to permit uses of function paramet=
ers
with empty types even outside of unevaluated contexts, since we don't really
care about the value of an empty type.=