subclass - Nested subclasses in C++ -

i'm trying make nested class subclass of parent:

struct x { struct y : public x {}; }; 

unfortunately, doesn't seem allowed in c++, g++ produces error

error: invalid use of incomplete type 'struct x'

however, actual code has x templated class:

template<typename t> struct x { struct y : public x {}; }; 

i same message, time it's warning:

warning: invalid use of incomplete type 'struct x< t >'

my question is: why former case illegal, while templated case gives warning? templated version works expect (i can create instances of x<t>::y, cast them x<t>, , on), warning mean shouldn't use it? problems can expect run if ignore warning?

technically, far compiler concerned, layout of base (x) not need known until template (x) instanciated. , template (x) may not istantiated before defined. @ point, it's layout known.

simplest way error template try istantiate y inside x:

template<typename t> struct x {     struct y : public x {};     y y; }; 

in earlier versions of complier, there no warning in case show, added @ point. here discussion gcc bugtracker whether warning spurious. there uncertainty whether allowed standard conclusion not allowed.

so, neither case allowed standard, gcc keeps working latter, because can.

yam marcovic shows how x::y can defined in standard compliant way. analoguously identical example shown in gcc bugtracker.