FBIs with Date columns and case option

Hi

Based on this link https://jonathanlewis.wordpress.com/category/oracle/indexing/function-based-indexes/ 

And trying to understand the requirement of one of my developer Could we have a FBI with case options for Dates nulls and Date not nutll?

Could we create a FBIs with Date columns and case options

create table t1 

( 

    col1    int not null,

    col2    varchar2(1)

); 

create unique index t1_i1 on t1(

--  case col2 when null then cast(null as int) else col1 end,

--  case when col2 is null then cast(null as int) else col1 end,

    case when col2 is not null then col1 end,