HAVING
            
When calculating GROUP BY the server caches some expressions. It does
that by allocating a string slot (Item_copy_string) and assigning the 
value of the expression to it. This effectively means that the result
type of the expression can be changed from whatever it was to a string.
As this substitution takes place after the compile-time result type 
calculation for IN but before the run-time type calculations, 
it causes the type calculations in the IN function done at run time 
to get unexpected results different from what was prepared at compile time.
                  
In the CASE ... WHEN ... THEN ... statement there was a similar problem
and it was solved by artificially adding a STRING argument to the set of 
types of the IN/CASE arguments at compile time, so if any of the 
arguments of the CASE function changes its type to a string it will 
still be covered by the information prepared at compile time.