`3:::Int` parses as `3::(:Int)`

```julia-repl
julia> 3:::Int
ERROR: TypeError: in typeassert, expected Type, got a value of type Symbol
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1

julia> 3::(:Int)
ERROR: TypeError: in typeassert, expected Type, got a value of type Symbol
```

Perhaps a warning/syntax error would make more sense.