Good work, but please add a test for the error.

Should not it be in the checker, instead of the parser?

The problem with it being in the parser, is that the parser has limited type information.
If the types are defined in another .v file in the same module, it will trigger different behaviour, when the check is done in the parser, depending on whether the other .v files are already parsed or not.

If the check is done in the checker, that is much less likely to happen, since everything is already parsed, and the checker will make sure that its analysis is done based on the full type information from all declarations, from all files.

Should not it be in the checker, instead of the parser?

Yeah, I moved to the parser because on checker the .typ was already resolved to []Callback[T] even when it was used []Callback.