[Bug libstdc++/52931] New: std::hash shouldn't be defined for unknown types

[marc.glisse at normalesup dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52931

             Bug #: 52931
           Summary: std::hash shouldn't be defined for unknown types
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: marc.gli...@normalesup.org


As explained by Daniel Krügler in c++std-lib-32420 and nearby messages, the
default definition of std::hash is non-standard, it is supposed to be undefined
so we can test with sfinae whether hash was specialized for some type.