Why collation doesn’t work with for xml path in subquery?

I usually use collate SQL_Latin1_General_CP1251_CI_AS to remove accents (diacritics) like this.

select ('ěščřžýáíé' + '-' + (select 'ěščřžýáíé')) collate SQL_Latin1_General_CP1251_CI_AS

escrzyaie-escrzyaie

Unfortunately, after one of the search queries stopped working, I found out that collation is ignored when used in combination with for xml path in subquery.

select ('ěščřžýáíé' + '-' + (select 'ěščřžýáíé' for xml path(''))) 
       collate SQL_Latin1_General_CP1251_CI_AS

ěščřžýáíé-ěščřžýáíé

It is possible to work around this by using collation for each part separately, but I wonder why it doesn’t work in the previous example.

select ('ěščřžýáíé' collate SQL_Latin1_General_CP1251_CI_AS + '-' 
     + (select 'ěščřžýáíé' collate SQL_Latin1_General_CP1251_CI_AS for xml path('')))

escrzyaie-escrzyaie

Answer

Cast it to varchar before concatenating

     select ('ěščřžýáíé' + '-' 
             + cast((select 'ěščřžýáíé' for xml path('')) as varchar(max))) 
               collate SQL_Latin1_General_CP1251_CI_AS