[qayxc]

They are obvious to the programmer who knows the types of b and d and thus the semantics of adding them.

The operation itself implies that both b and c.d must be primitive types and so the semantics of the operation are defined by Zig's language rules.

edit: To clarify - this in contrast to, say C++, where nothing can be inferred about the types of the variables involved and the semantics of the operation, since '+' can be overloaded.