foo.nim

static:
    doAssert int is int
    doAssert type(1) is int
    # doAssert string is int # <- compile time error


match(either) # Either[string, int]
of int: ...
of string: ...
of bool:
    static: # this block would be generated by the macro
            # and inserted in each branch
        var found = false
        found = found or (bool is string) # first type is the ident, second is 
        found = found or (bool is int)    # the properly typed sym extracted in your macro
        if not found:
            error("Bool is not a valid type!")
    # user branch code goes here...