https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683
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=c++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 = Inner;
<template-parameter-1-2> = 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 = Inner;
<template-parameter-1-2> = 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 = Inner]
<source>:4:14: error: satisfaction of atomic constraint
'requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);}
[with T = 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
