Cast specification evaluation is not compliant with the SQL standard. Mainly, it yields errors for standard-format strings that are specified to successfully yield interval values.
In ISO/IEC 9075-2:2011(E) section 6.13 <cast specification>, General Rule 19 case b says that, in a <cast specification> casting to an interval type, a character string value that is a valid interval <literal> (<interval literal>) or <unquoted interval string> yields an interval value.
(<interval literal> is the "INTERVAL '1-6' YEAR TO MONTH" syntax; <unquoted interval string> is the "1-6" syntax.)
Drill currently rejects both of those syntaxes. Note the casts to type INTERVAL HOUR and the resulting error messages in the following:
(The extra cast to VARCHAR is a workaround for a CHAR-vs.-VARCHAR bug.)
Drill should accept the standard formats or at least document the non-compliance for users.