The opaque keyword in Zig doesn't support method definitions - it creates an incomplete type that must be implemented elsewhere, not a type that can have inline methods and fields like your example attempts.

https://ziglang.org/documentation/master/#opaque

> opaque {} declares a new type with an unknown (but non-zero) size and alignment. It can contain declarations the same as structs, unions, and enums

So it can contain methods, just like structs. The only thing it cannot contain is fields (which the example above doesn't contain). An opaque type can only be used as a pointer, it's like a `typedef void* ...` in C but with the possibility to add declarations within a namespace

Edit: the documentation doesn't contain any example of an opaque type which happens to have declarations inside. But here is one: https://codeberg.org/andrewrk/daw/src/commit/38d3f0513bf9bfc...