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From: "dani at danielbertalan dot dev" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109683] New: [13/14 Regression] False cyclic dependency
 error reported for constraint
Date: Mon, 01 May 2023 09:43:36 +0000
X-Bugzilla-Reason: CC
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.1.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: dani at danielbertalan dot dev
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109683

            Bug ID: 109683
           Summary: [13/14 Regression] False cyclic dependency error
                    reported for constraint
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dani at danielbertalan dot dev
  Target Milestone: ---

The following code (reduced from a custom std::variant-like type) compiles =
with
GCC 12.2 and Clang, but is rejected by GCC 13+:

```
template <typename T, typename>
struct VariantConstructors {
  VariantConstructors(T&& t)
    requires(requires { T(t); });
};

template <typename... Ts>
struct InheritFromEntries : Ts... {};

template <typename... Ps>
struct InheritFromPack: InheritFromEntries<Ps>... {
  using InheritFromEntries<Ps>::InheritFromEntries...;
};

template <typename... Ts>
struct Variant : InheritFromPack<VariantConstructors<Ts, Variant<Ts...>>...>
{};

template <typename T>
class Outer;
struct Inner {
    Inner(Outer<int>);
};
template<typename T>
class Outer {
    Variant<T, Inner> value_;
};

struct Empty {};
void fn(Outer<Empty> x) {}
```

The following compiler error is produced (arguments: -std=3Dc++20):

<source>: In instantiation of 'VariantConstructors<T, <template-parameter-1=
-2>
>::VariantConstructors(T&&) requires requires{T(VariantConstructors<T,
<template-parameter-1-2> >::__ct ::t);} [with T =3D Inner;
<template-parameter-1-2> =3D Variant<int, Inner>]':
<source>:12:51:   required from 'struct
InheritFromPack<VariantConstructors<int, Variant<int, Inner> >,
VariantConstructors<Inner, Variant<int, Inner> > >'
<source>:16:8:   required from 'struct Variant<int, Inner>'
<source>:25:23:   required from 'class Outer<int>'
<source>:4:25:   required from 'VariantConstructors<T, <template-parameter-=
1-2>
>::VariantConstructors(T&&) requires requires{T(VariantConstructors<T,
<template-parameter-1-2> >::__ct ::t);} [with T =3D Inner;
<template-parameter-1-2> =3D Variant<Empty, Inner>]'
<source>:12:51:   required from 'struct
InheritFromPack<VariantConstructors<Empty, Variant<Empty, Inner> >,
VariantConstructors<Inner, Variant<Empty, Inner> > >'
<source>:16:8:   required from 'struct Variant<Empty, Inner>'
<source>:25:23:   required from 'class Outer<Empty>'
<source>:29:23:   required from here
<source>:3:3:   required by the constraints of 'template<class T, class>
VariantConstructors<T, <template-parameter-1-2> >::VariantConstructors(T&&)
requires requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct
::t);}'
<source>:4:14:   in requirements  [with T =3D Inner]
<source>:4:14: error: satisfaction of atomic constraint
'requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);}
[with T =3D T]' depends on itself
    4 |     requires(requires { T(t); });
      |             ~^~~~~~~~~~~~~~~~~~~

Reproducer on Compiler Explorer: https://godbolt.org/z/TbcoanG5T
The non-reduced preprocessed source can be found here:
https://godbolt.org/z/69dMMoWKh=