---- Reported by marek.sieradzki@gmail.com 2007-02-03 13:07:57 MST ----

Description of Problem:
Comparison between a struct (not nullable) and null causes compilation error.

Steps to reproduce the problem:
1. Compile attached test case.

Actual Results:
struct.cs(18,22): error CS0019: Operator `==' cannot be applied to operands
of type `A' and `null'

Expected Results:
Compilation succeeded (not really sure because it's such a weird case)

How often does this happen? 
Always

Additional Information:
Compiled code prints 'false' with CSC 2.0



---- Additional Comments From marek.sieradzki@gmail.com 2007-02-03 13:08:39 MST ----

Created an attachment (id=171388)
test case (.cs)




---- Additional Comments From miguel@ximian.com 2007-02-14 14:57:47 MST ----

This seems to be a bug in csc, not a bug in mcs.

CSC 1 reports that error correctly, we went through this recently. 
CSC 2 will basically generate a constant for that compare (as structs
can never be null) and hide real bugs in your code.

So I would say this is not a bug.



---- Additional Comments From gert.driesen@pandora.be 2007-02-14 15:18:51 MST ----

I've reported this issue to MS using the connect web site:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?
FeedbackID=257924

We'll see what MS responds on this.



---- Additional Comments From jonpryor@vt.edu 2007-04-17 11:52:48 MST ----

Microsoft's Response:

The difference between the 1.1 compiler and the 2.0 compiler here is
actually due to a change in the spec which makes the code you provided
legal. This is related to the new C#2.0 feature "Nullable value types".

Any operator between struct types, including ==, can now be 'lifted'
to apply to the nullable version of the types. In your case, the ==
operator from a pair of As to bool can be lifted to an operator from a
pair of "A?"s to bool. The left hand argument is implicitly converted
from A to A? and the right hand "null" is convertible to A? be
default. The specific operation in your code will always return
"false" though.




---- Additional Comments From miguel@ximian.com 2007-07-23 13:17:35 MST ----

I think the explanation from Microsoft is incomplete, because in the
particular case of:

class Foo<T> where T: struct {

     bool IsNull (T x) { return x == null; }
}


Is not permitted.   If you remove "struct" then the above code
compiles with CSC. 

Imported an attachment (id=171388)

Unknown operating system unknown. Setting to default OS "Other".