[Bug c++/109683] New: [13/14 Regression] False cyclic dependency error reported for constraint

[Bug c++/109683] New: [13/14 Regression] False cyclic dependency error reported for constraint

[dani at danielbertalan dot dev]

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

[Bug c++/109683] [13/14 Regression] False cyclic dependency error reported for constraint

[ali.mpfard at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683

Ali Mohammad Pur Fard <ali.mpfard at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ali.mpfard at gmail dot com

--- Comment #1 from Ali Mohammad Pur Fard <ali.mpfard at gmail dot com> ---
As a note, using a type alias for `T` in the constraint seems to work around
the issue:
```
template <typename T, typename>
struct VariantConstructors {
  using U = T;
  VariantConstructors(T&& t)
    requires(requires { U(t); });
};

// Everything below is the same as the repro case

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) {}
```

(https://godbolt.org/z/1GTrjcvG6)

[Bug c++/109683] [13/14 Regression] False cyclic dependency error reported for constraint

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=105797
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2023-05-01
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid
   Target Milestone|---                         |13.2
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report.  Started with r13-980-gdf4f95dbd4764f which made
function parameters used in a constraint no longer induce a dependency on all
contextual template parameters.

So another workaround would be to make the constructor's constraints explicitly
depend on the second template parameter of the class:

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

...

[Bug c++/109683] [13/14 Regression] False cyclic dependency error reported for constraint

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/109683] [13/14 Regression] False cyclic dependency error reported for constraint

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.2                        |13.3

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.2 is being released, retargeting bugs to GCC 13.3.