---- Reported by ben@joldersma.org 2006-11-19 13:43:24 MST ----

Description of Problem:
When trying to use a generic type in a derived generic method that is
constrained in the base method, it is not possible to perform at least two
expected kinds of operations, both related to constraint specifications.  

The first is that if the base method is constrained with the public
parameterless constructor (aka 'new()') constraint, it is not possible to
use that type in the derived method to call another generic method with the
'new()' constraint.  It is possible to initialize a variable of the type T
and assign to the public parameterless constructor.

The second bug I am seeing, probably the same bug, is that if you have a
class constraint (say class Foo, with public property X) in the base
method, you cannot access the property in derived method (although you can
if the constraint is declared in the calling method, rather than a derived
one.)

Steps to reproduce the problem:

Try to compile the following code using the mono 1.2 compiler (I am
compiling inside of the MonoDevelop that is included with the mono 1.2
generic linux install)

//BEGIN CODE FILE
using System;

namespace testConstraintsVirtuals
{
	class MainClass
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Hello World!");
		}
	}
	
	class Foo
	{
		public int X;
	}
	
	abstract class Base
	{
		public abstract void method<T>() where T : Foo, new();
	}
	
	class Derived : Base
	{
		// the first method signature is the ideal one.  this one compiles with
ms csc.  it's what you'd expect to do.
		// in mono 1.1.17, you could remove the override, and specify
constraints, and it would compile, now no longer.
		// gives error: The type `T' must have a public parameterless constructor
in order to use it as parameter `T' in the generic type or method
`testConstraintsVirtuals.Derived.method2<T>()'(CS0310)
		public override void method<T>()

		// public override void method<T>() where T : new()
		// gives error: Cannot specify constraints for overrides or explicit
interface implementation methods
			
		// public void method<T>() where T : new()
		// gives error: testConstraintsVirtuals.Derived.method<T>()' hides
inherited abstract member
		
		// new public void method<T>() where T : new()
		// gives error: testConstraintsVirtuals.Derived.method<T>()' hides
inherited abstract member
		{
			method2<T>(); // cannot see that T is constrained to new()
			T t = new T(); //works as expected 
			System.Console.WriteLine(t.X); //cannot see that T is constrained to Foo.
		}
		
		public void method2<T>() where T : Foo, new()
		{
			T t = new T();  //works as expected
			System.Console.WriteLine(t.X); //works as expected
		}
	}
}
//END FILE

Actual Results:
compiler gives these errors:

Line 39: The type `T' must have a public parameterless constructor in order
to use it as parameter `T' in the generic type or method
`testConstraintsVirtuals.Derived.method2<T>()'(CS0310)

and

Line 41: `T' does not contain a definition for `X'(CS0117)

Expected Results:

to successfully compile, and correctly inherit the constraints.

How often does this happen? 

every time.

Additional Information:

NA



---- Additional Comments From martin@ximian.com 2007-03-15 14:45:01 MST ----

Fixed in SVN.


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