Curious inconsistent behaviour from SQL Server in windowed function clauses?

Question

Whilst asking another question, I discovered that SQL Server (happens both in 2005 and 2008) seems to have strange inconsistent behaviour when dealing with CASE

Answer 1

The evaluations in your first example will never change.

You are comparing a constant to a constant which will constantly result in a constant.

1=1 will always be TRUE.
1=0 will always be FALSE.

Answer 2

Books online indicates that "A sort column can include an expression, but when the database is in SQL Server (90) compatibility mode, the expression cannot resolve to a constant." however it does not define "constant".

From thinking about it and some experimentation it seems clear that this means an expression for which a literal constant value can successfully be calculated at compile time.

/*Works - Constant at run time but SQL Server doesn't do variable sniffing*/
DECLARE @Foo int
SELECT ROW_NUMBER() OVER (ORDER BY @Foo) 
FROM master..spt_values 

/*Works - Constant folding not done for divide by zero*/
SELECT ROW_NUMBER() OVER (ORDER BY $/0) 
FROM master..spt_values 

/*Fails - Windowed functions do not support 
   constants as ORDER BY clause expressions.*/
SELECT ROW_NUMBER() OVER (ORDER BY $/1) 
FROM master..spt_values