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From: "hr.jonas.hansen at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108536] New: Difference when using requires and enable_if
 with class constructor
Date: Wed, 25 Jan 2023 09:44:48 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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X-Bugzilla-Who: hr.jonas.hansen at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108536

            Bug ID: 108536
           Summary: Difference when using requires and enable_if with
                    class constructor
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hr.jonas.hansen at gmail dot com
  Target Milestone: ---

In the code below I have used a requires-clause. This requires-clause used =
to
be an enable_if. When using enable_if the code compiles without errors, but
using the requires-clause (see below) causes a compilation error when combi=
ned
with the rest of the example. That is, the example contains two classes Cla=
ssA
and ClassB. If either of the classes ClassA and ClassB are removed then the
code compiles without errors.


Compile with: g++ -std=3Dc++20 example.cpp



#include <type_traits>

struct Base {
    Base() noexcept =3D default;

    template <typename F, typename DecayF =3D std::decay_t<F>>
    // If this requires-clause is replaces with an enable_if then the code
compiles fine
        requires(!std::is_same_v<DecayF, Base>
                 && std::is_constructible_v<DecayF, F>)
    Base(F&&) {}
};

struct Derived : public Base {
    using Base::Base;
    void operator()() const;
};

class ClassA {
    // The class ClassB must be present for the bug to manifest
    class ClassB;

    // This is the only usage of 'Derived'
    Derived const f;
};

// This class and its contructor must be included for the bug to manifest
class ClassA::ClassB {
    ClassB();
};=