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From: "ich.freak at gmx dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/103540] diagnosting concept depends on itself
Date: Sun, 05 Dec 2021 16:04:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103540

--- Comment #2 from Igel <ich.freak at gmx dot net> ---
Ah thanks for copying the relevant code here. The error that you're referri=
ng
to is:

<source>: In instantiation of 'struct X<0>':
<source>:5:14:   required by substitution of 'template<class N>  requires=20
StrictNodeType<N> using Int =3D int [with N =3D X<0>]'
<source>:15:27:   required from here
<source>:12:11: error: template constraint failure for 'template<class N>=20
requires  StrictNodeType<N> using Int =3D int'
   12 |     using something =3D Int<X>;
      |           ^~~~~~~~~
<source>:12:11: note: constraints not satisfied


I guess technically, you're right that it points to the correct spot and it=
 is
the programmers duty to translate this into
"the concept StrictNodeType<X<0>> depends on itself because the type
X<0>::something (also known as Int<X<0>>) declared in line 12 depends on the
concept StrictNodeType<X<0>>".
However, this information can be buried arbitrarily deep in the error messa=
ges.
Consider, for example, the following adaptation:

#include <type_traits>

template<typename N>
concept StrictNodeType =3D requires {
    typename N::something;
};

template<StrictNodeType N> using Int =3D int;=20

template<template<class> class P, class Q>
using chooser =3D P<Q>;

template<int>
struct X { using something =3D chooser<Int, X>; };

using ThisBreaks =3D Int<X<0>>;


The only hint towards the real problem (gcc cannot tell if X<0> has a type
called 'something' or not) is now this:
<source>:14:18:   required from 'struct X<0>'

I guess all I'm saying is that extracting the site of the problematic code =
is
currently very hard for "depends on itself"-errors.=