conflicts between _ and named tokens not detected

[nikomatsakis]

The following grammar has a conflict between two regexs that is flagged:

Test = {
    r"[a-z]*",
    r"[a-z]*"
};

which is flagged

  processing file `/home/nmatsakis/versioned/duchess/src/class_info/javap_parser.lalrpop`
  /home/nmatsakis/versioned/duchess/src/class_info/javap_parser.lalrpop:140:5: 140:13 error: ambiguity detected between the terminal `r#"[a-z]*"#` and the terminal `r#"[a-zA-Z][a-zA-Z0-9]*"#`

  --- stderr
        r"[a-z]*",
        ~~~~~~~~~

but if I use a match with _...

Test = {
    r"[a-z]*",
    ID
};

match {
    r"[a-z]*" => ID,
    _
}

...then I get no errors. This can produce some really confusing failures at parse time.