[LLVMbugs] [Bug 13052] New: explicitly defaulted constructor is not constexpr

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13052

             Bug #: 13052
           Summary: explicitly defaulted constructor is not constexpr
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: other
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: eric at boostpro.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Clang rejects the following code:

   template<typename T>
   struct wrap
   {
       // Either delete this line or add constexpr
       // to make clang happy.
       wrap(wrap &&) = default;

       T t;

       template<typename U>
       explicit constexpr wrap(U &&u)
         : t(static_cast<U &&>(u))
       {}
   };

   template<typename T>
   struct S
   {
       S(S &&) = default;

       T t;

       template<typename U>
       explicit constexpr S(U &&u)
         : t(static_cast<U &&>(u))
       {}
   };

   constexpr S<wrap<int>> s0(42); // OK
   constexpr S<wrap<S<wrap<int>>>> s1(S<wrap<int>>(42)); // FAIL

The comments in the wrap struct shed more light on the issue. According to
Richard Smith on the ISO core reflector, this is a bug in clang.
[dcl.fct.def.default]p2 says:

"If a function is explicitly defaulted on its first declaration, it is
implicitly considered to be constexpr if the implicit declaration would be"

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