c++ - Can SFINAE detect private access violations?

Question

I wonder whether if i test for some member of a class and the member is private what will sfinae respond? Will it error out hard or will it say ok or will it error out in the sfinae way?

Answer 1

Yes.

EDIT: C++11 Standard quote from §14.8.2 [temp.deduct]

8/ If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed if written using the substituted arguments. [ Note: Access checking is done as part of the substitution process. —end note ]

This suggests to me that private can trigger an SFINAE error. Reading on:

Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure. [ Note: The evaluation of the substituted types and expressions can result in side effects such as the instantiation of class template specializations and/or function template specializations, the generation of implicitly-defined functions, etc. Such side effects are not in the “immediate context” and can result in the program being ill-formed.—end note ]

The "immediate context" is not so clear to me... but it does not contradict my point :)

end of EDIT

So it seems to me that it will error out in an SFINAE way, this is further confirmed by this excerpt from Clang:

// clang/Basic/DiagnosticIDs.h:185-209

  /// rief Enumeration describing how the the emission of a diagnostic should
  /// be treated when it occurs during C++ template argument deduction.
  enum SFINAEResponse {
    /// rief The diagnostic should not be reported, but it should cause
    /// template argument deduction to fail.
    ///
    /// The vast majority of errors that occur during template argument
    /// deduction fall into this category.
    SFINAE_SubstitutionFailure,

    /// rief The diagnostic should be suppressed entirely.
    ///
    /// Warnings generally fall into this category.
    SFINAE_Suppress,

    /// rief The diagnostic should be reported.
    ///
    /// The diagnostic should be reported. Various fatal errors (e.g.,
    /// template instantiation depth exceeded) fall into this category.
    SFINAE_Report,

    /// rief The diagnostic is an access-control diagnostic, which will be
    /// substitution failures in some contexts and reported in others.
    SFINAE_AccessControl
  };

There are special cases with regard to Access Control in the case of SFINAE.