[Bug libstdc++/51558] Declaration of unspecialized std::hash<_Tp>::operator()(_Tp) turns compile-time errors into link-time errors

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51558

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-15 
02:01:24 UTC ---
Would the condition of the static_assert need to depend on the template
parameter somehow, to avoid the case where no valid specialization can ever be
generated? (Which would be ill-formed, no diagnostic required, by [temp.res]
p8.)

e.g.

static_assert( integral_constant<int, sizeof(_Tp)>::value,
  "std::hash must be specialized for this type" )