Incomplete type is not allowed in a class, but is allowed in a class template

Question

The following is invalid code:

struct foo {
    struct bar;
    bar x;        // error: field x has incomplete type
    struct bar{ int value{42}; };
};

int mai         


        

Answer 1

I'll answer the third part of your question - as IANALL (not a language lawyer).

The code is invalid for the same reason it's invalid to use a function before it has been declared - even though the compiler can figure out what the function's supposed to be by going further down in the same translation unit. And the cases are similar also in the sense that if you happen to have just a declaration with no definition, that's good enough for the compiler, while here you happen to have a template definition before the instantiation.

So the point is: The language standard mandates that the compiler does not look ahead for you when you want to define something (and a class template is not a definition of a class).