CRTP — accessing incomplete type members

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When a class template is instantiated, its members other than non-virtual member functions are instantiated together with it. Non-virtual member functions, however, are only instantiated when odr-used (basically, called or have their address taken).

When the compiler encounters class Implementation : public Interface<Implementation>, it needs to instantiate Interface<Implementation>. At this point, Implementation is still an incomplete type, its TYPE member has not yet been seen. On the other hand, Interface<Implementation>::foo is only instantiated later, when it’s called in main. At that point, Implementation is a complete type.

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