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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107179] New: requires-expression gives hard error for
 inaccessible constructor
Date: Fri, 07 Oct 2022 11:53:12 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.1
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107179

            Bug ID: 107179
           Summary: requires-expression gives hard error for inaccessible
                    constructor
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<bool B> struct bool_constant { static constexpr bool value =3D B; =
};

template<typename T>
  struct is_implicitly_default_constructible
  : bool_constant<requires (void (*f)(const T&)) { f({}); }>
  { };

class X {
  X();
public:
  X(int);
};

static_assert( !is_implicitly_default_constructible<X>::value );


This should compile with -std=3Dc++20 but doesn't:

conv.cc: In instantiation of 'struct is_implicitly_default_constructible<X>=
':
conv.cc:14:55:   required from here
conv.cc:5:53: error: 'X::X()' is private within this context
    5 |   : bool_constant<requires (void (*f)(const T&)) { f({}); }>
      |                                                    ~^~~~
conv.cc:9:3: note: declared private here
    9 |   X();
      |   ^
conv.cc:14:16: error: static assertion failed
   14 | static_assert( !is_implicitly_default_constructible<X>::value );
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~=