This commit adds generic variance support under the new

"future" langversion. Variance is allowed in interface
and delegate types. Covariant type parameters are denoted
with the "out" keyword like so:

interface IFoo<out T> {
    T Bar { get; }
}

And contravariant type parameters are dentoed with the
"in" keyword, like so:

delegate void Bar<in T> (T input);

Covariant type parameters can only be used as method
return types and type arguments for inherited
interfaces. Contravariant type parameters can only be
used as by-value method parameter types.

Only reference type are variant.

!!!IT SHOULD BE NOTED!!!
That the Mono VM does not yet fully support generic
type variance, so variant code compiled with this
feature will not run on Mono until variance support
is complete. Code compiled with this feature will run
on the .NET 2.0 virtual machine.

This patch is contributed under the MIT/X11 license.


svn path=/trunk/mcs/; revision=125647

1 files changed, 34 insertions, 0 deletions

diff --git a/mcs/tests/gtest-variance-1.cs b/mcs/tests/gtest-variance-1.cs
new file mode 100644
index 00000000000..04fac5216fd
--- /dev/null
+++ b/mcs/tests/gtest-variance-1.cs
@@ -0,0 +1,34 @@
+// Compiler options: -langversion:future
+
+interface IFoo<out T>
+{
+	T Bar { get; }
+}
+
+class Foo : IFoo<string>
+{
+	readonly string bar;
+	public Foo (string bar)
+	{
+		this.bar = bar;
+	}
+	public string Bar { get { return bar; } }
+}
+
+public class Test
+{
+	static int Main ()
+	{
+		string bar = "Who is John Galt?";
+		IFoo<string> foo = new Foo(bar);
+		IFoo<object> foo2 = foo;
+		if (!foo2.Bar.Equals (bar))
+			return 1;
+
+		foo2 = new Foo(bar);
+		if (foo2.Bar != bar)
+			return 2;
+
+		return 0;
+	}
+}