ue-trefcountptr-member-needs-complete-type

TRefCountPtr<T> / TSharedPtr<T> members need T's complete type

When to use

You added a data member like this to a UE module header:

// Pb4ueTrpcServerConnection.h
class FPb4ueTrpcServerStream;           // forward decl

class FPb4ueTrpcServerConnection : public TSharedFromThis<...>
{
    ...
    TMap<uint32, TRefCountPtr<FPb4ueTrpcServerStream>> ServerStreams;
};

The header itself parses. Then the first TU that constructs the enclosing class — typically via MakeShared<FPb4ueTrpcServerConnection>() or a TUniquePtr<FPb4ueTrpcServerConnection> reset — fails to compile with errors pointing at the smart pointer's destructor:

error: invalid application of 'sizeof' to an incomplete type 'FPb4ueTrpcServerStream'
note: in instantiation of member function 'TRefCountPtr<FPb4ueTrpcServerStream>::~TRefCountPtr' requested here
note: in instantiation of member function 'TMap<unsigned int, TRefCountPtr<FPb4ueTrpcServerStream>>::~TMap' requested here
note: in instantiation of member function 'FPb4ueTrpcServerConnection::~FPb4ueTrpcServerConnection' requested here

Same pattern with TSharedPtr<T>, TUniquePtr<T>, and TArray<TRefCountPtr<T>>.

Problem

TRefCountPtr<T>, TSharedPtr<T>, TUniquePtr<T> all call into T's destructor from their own destructor. That requires T to be a complete type at the point the smart pointer's destructor is instantiated. When the smart pointer is a class member, the enclosing class's implicit destructor instantiates the member destructors, so the completeness requirement propagates out to every TU that destroys the enclosing class — including the one that calls MakeShared<...>().

A forward declaration in the header is enough to parse the member declaration (you're only taking a pointer-sized field), but not enough to destroy it. The compiler doesn't fire at the header; it fires at the first caller that forces instantiation.

This is the same root cause as the classic "pimpl needs out-of-line destructor" rule, but UE's TRefCountPtr has no std::unique_ptr-style defaulted-dtor escape hatch, so the workaround is narrower.