[Bug ipa/61659] [4.9/4.10 Regression] Extra undefined symbol because of devirtualization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61659

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Markus Trippelsdorf from comment #10)
> Created attachment 33036 [details]
> testcase

I think this is a correct error message.  See
https://gcc.gnu.org/gcc-4.9/porting_to.html at the end.
GCC now checks return types more strictly and will reject declarations of
functions which return abstract types, including in uninstantiated templates
and in typedefs to function pointers. Returning an abstract type is not
possible so the code must be fixed.

Also see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60775#c4:
14.7.1 [temp.inst] p1:
"The implicit instantiation of a class template specialization causes the
implicit
instantiation of the declarations, but not of the definitions, default
arguments, or exception-specifications of the class member functions, member
classes, scoped member enumerations, static data members and member templates;"