[Mono-dev] Generic Constraints Regression in 2.8

[Bojan Rajkovic]

On Nov 14, 2010, at 10:16 PM, Tom Philpot wrote:

> The following code compiled with a warning on Mono 2.7 (as of 147679)  but has  an error in 2.8.
> 
> namespace Application
> {
> 	public class Z {}
> 
> 	public class A<X, Y>
> 		where Y : Z
> 		where X : Y
> 	{
> 		protected void SomeOperation(Y y)
> 		{
> 			X x = y as X;
> 		}
> 	}
> 
> 	public class Foo { public static void Main() {} }
> }
> 
> /opt/mono-2.8/bin/gmcs CS0413Regression.cs
> CS0413Regression.cs(12,33): error CS0413: The `as' operator cannot be used with a non-reference type parameter `X'. Consider adding `class' or a reference type constraint
> Compilation failed: 1 error(s), 0 warnings
> 
> /opt/mono-r147679/bin/gmcs CS0413Regression.cs
> CS0413Regression.cs(12,27): warning CS0219: The variable `x' is assigned but its value is never used
> Compilation succeeded - 1 warning(s)

The error message here is the correct thing to do—as casts are only valid with reference types, because they can result in null, which is invalid for a value type. If this raised only a warning in gmcs earlier, it was a bug.

—Bojan

> 
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