Q:

Find all constraints that are referenced by another constraint

Meldung 3725, Ebene 16, Status 0, Zeile 327
The constraint 'UQ_users_email' is being referenced by table 'posts', foreign key constraint 'FK_posts_users'. 
Meldung 3727, Ebene 16, Status 0, Zeile 327
Could not drop constraint.  See previous errors. 

Besides parsing the error message, how can I get a list of all constraints that block the drop of a certain constraint? 

A:

FK depends on (blocks drop of) PK/UQ of referenced table, any kind of constraint blocks drop of column(s) it depends on.  List of tables referencing target table can be listed by this query:
select
  object_schema_name(fk.fkeyid) + '.'  + object_name(fk.fkeyid) as [REF_BY], 
  object_name(fk.constid) as [REF_NAME]
from sys.sysforeignkeys fk
inner join sys.sysconstraints cs on cs.constid = fk.constid
where fk.rkeyid = object_id('dbo.my_target_table_with_constraint_to_drop', 'u')
order by 1, 2

all these FKs will block drop of PK/UQ from target table

