Minimal reproduction:
fn fun(a: T...) void;
export fn main() void = fun();
The issue is in check.c:1479 (and a bit lower for no arguments case), where param->type can be have storage=STORAGE_ERROR if the type is not resolved, but this is not checked and the code assumes that the type is slice and gets a NULL pointer for the member type that is later dereferenced during type lookup.
Alexey Yerin referenced this ticket in commit b256a32.