Which is why I was saying it should be decompiled down to it's logical equivalent, which will do the right thing:

create table #t (fname varchar(20))

insert #t (fname)

select top 100 'asdf'

from sysobjects

insert #t (fname) values ('zzzz')

create index a on #t (fname)

declare @fname varchar(20)

set @fname = 'asdf' --does index scan

select *

from #t

WHERE

CASE WHEN fname = @fname

THEN 1

ELSE 0

END = 1

dbcc freeproccache

go

declare @fname varchar(20)

set @fname = 'zzzz' --does index scan as well, even though only 1 row

select *

from #t

WHERE

CASE WHEN fname = @fname

THEN 1

ELSE 0

END = 1

dbcc freeproccache

go

declare @fname varchar(20)

set @fname = 'asdf' --does index seek, since covering and ordered

select fname

from #t

WHERE fname = @fname

dbcc freeproccache

go

declare @fname varchar(20)

set @fname = 'zzzz' --does index seek, since 1 row

select fname

from #t

WHERE fname = @fname