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.