---- Reported by atsushi@ximian.com 2005-09-16 16:48:37 MST ----

The example below fails to compile because "this" is regarded as
incompatible with T. However T might be Test so it should pass.

using System.Collections.Generic;

class Test
{
        public IEnumerable<T> Foo <T> (IEnumerator<object> e)
        {
                List<T> l = new List<T> ();
                if (this is T)
                        l.Add ((T) this);
                return l;
        }
}

Actual Results:

$ !gmcs
gmcs generic-t.cs
generic-t.cs(8,12): warning CS0184: The given expression is never of the
provided (`T') type
generic-t.cs(9,12): error CS0030: Cannot convert type 'Test' to 'T'
Compilation failed: 1 error(s), 1 warnings

Expected Results:

No CS0030 error.

How often does this happen? 

consistently.

Additional Information:

MS csc also fails to compile it (the same CS0030 error).


Here are the corresponding sections from C# 2.1 spec. I think this "T" is
an open type.

--------
14.9.10 is operator
The is operator is used to dynamically check if the run-time type of an
object is compatible with a given
type. The result of the operation e is T, where e is an expression and T is
a type, is a Boolean value
indicating whether e can successfully be converted to type T by a reference
conversion, a boxing conversion,
or an unboxing conversion. The operation is evaluated as follows:
• If the compile-time type of e is the same as T, or if an implicit
reference conversion (§13.1.4) or boxing
conversion (§13.1.5) exists from the compile-time type of e to T:
o If e is of a reference type, the result of the operation is equivalent to
evaluating e != null.
o If e is of a value type, the result of the operation is true.
• Otherwise, if an explicit reference conversion (§13.2.3) or unboxing
conversion (§13.2.4) exists from
the compile-time type of e to T, a dynamic type check is performed:
o If the value of e is null, the result is false.
o Otherwise, let R be the run-time type of the instance referenced by e. If
R and T are the same type, if
R is a reference type and an implicit reference conversion from R to T
exists, or if R is a value type
and T is an interface type that is implemented by R, the result is true.
o Otherwise, the result is false.
• Otherwise, if the compile-time type of e or T is an open type (§25.5.2),
a dynamic type check is
performed.

--------
25.5.2 Open and closed types
All types can be classified as open types or closed types. An open type is
a type that involves type
parameters. More specifically:
• A type parameter defines an open type.



---- Additional Comments From atsushi@ximian.com 2005-09-18 06:14:43 MST ----

http://lab.msdn.microsoft.com/ProductFeedback/viewFeedback.aspx?FeedbackID=FDBK36922



---- Additional Comments From atsushi@ximian.com 2005-09-20 00:57:56 MST ----

It turned out to be invalid since there is cast expression at line 9,
not "is".


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