そうみたいだな
調べたらやっぱりCのstructとの違いだったわ
C allows “compatible types” in several places, C + + does not
For example, otherwise-identical struct types with different tag names are “compatible” in C but are distinctly
different types in C + +.

ついでにPodの記述
A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct,
non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator
and no user-defined destructor.