[LLVMbugs] [Bug 20002] New: Cannot derive from std::function

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20002

            Bug ID: 20002
           Summary: Cannot derive from std::function
           Product: libc++
           Version: 3.4
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: david_work at me.com
                CC: llvmbugs at cs.uiuc.edu, mclow.lists at gmail.com
    Classification: Unclassified

#include <functional>

struct s : std::function< void() > {
    using function::function;
};

s q( std::function< void() >( []{} ) );

Nothing says std::function can't or shouldn't be used as a base class. However,
this simple test yields an error,

c++/v1/functional:1454:41: error: no type named 'type' in
'std::__1::enable_if<false, void>'; 'enable_if' cannot be used to disable this
declaration

I'm not sure why SFINAE isn't allowed in that context, but it might be because
of the phrasing of 12.9/1.2:

— for each non-template constructor of X that has at least one parameter with a
default argument, the set of constructors that results from omitting any
ellipsis parameter specification and successively omitting parameters with a
default argument from the end of the parameter-type-list

Omitting the default argument strips SFINAE from the constructor. The fix is to
move the SFINAE condition to the exception specification.

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