consider the following (also on https://gcc.godbolt.org/z/6ikNyq):

structS; // (1)

structU {
     S* sp_;
     U(S* sp) : sp_{sp} {}
     autop() -> int;
};

structB {
     inti_ = 42;
     autop(inta) -> int{ returna + i_; }
};

structS : B { // (2) Type S is completed here ... legal to introduce B
as base?
     autop() -> int{ returnB::p(3); }
};

autoU::p() -> int{ returnsp_->p(); }

intmain() {
S s{};
U u{&s};
returnu.p();
}

Is this legal? Reading the standard I do not find anything that either
forbids it, but neither do I find anything that allows it.
All compilers that I tried are fine with it, but does anyone know
whether this is intentional?