A compiler bug (I'm fairly sure it doesn't match the specification that is.) in VS 2013 that causes unnecessary boxing:

_Resulting CIL code of TestMethod (compiled in Release mode with VS 2013 Community Version 12.0.31101.00 Update 4)

.method public hidebysig static 
	void TestMethod<(class IEnumerable`2<!!T, !!TEnumerator>) TEnumerable, (class [mscorlib]System.Collections.Generic.IEnumerator`1<!!T>) TEnumerator, T> (
		!!TEnumerable enumerable
	) cil managed 
{
	// Method begins at RVA 0x2050
	// Code size 70 (0x46)
	.maxstack 1
	.locals init (
		[0] !!TEnumerator
	)

	IL_0000: ldarga.s enumerable
	IL_0002: constrained. !!TEnumerable
	IL_0008: callvirt instance !1 class IEnumerable`2<!!T, !!TEnumerator>::GetEnumerator()
	IL_000d: stloc.0
	.try
	{
		IL_000e: br.s IL_001e
		// loop start (head: IL_001e)
			IL_0010: ldloca.s 0
			IL_0012: constrained. !!TEnumerator
			IL_0018: callvirt instance !0 class [mscorlib]System.Collections.Generic.IEnumerator`1<!!T>::get_Current()
			IL_001d: pop

			IL_001e: ldloca.s 0
			IL_0020: constrained. !!TEnumerator
			IL_0026: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
			IL_002b: brtrue.s IL_0010
		// end loop

		IL_002d: leave.s IL_0045
	} // end .try
	finally
	{
		IL_002f: ldloc.0
		IL_0030: box !!TEnumerator // This is the unnecessary boxing that violates the specification.
		IL_0035: brfalse.s IL_0044

		IL_0037: ldloca.s 0
		IL_0039: constrained. !!TEnumerator
		IL_003f: callvirt instance void [mscorlib]System.IDisposable::Dispose()

		IL_0044: endfinally
	} // end handler

	IL_0045: ret
} // end of method Test::TestMethod

IEnumerable.cs

using System.Collections.Generic;

public interface IEnumerable<T, TEnumerator> : IEnumerable<T>
{ new TEnumerator GetEnumerator(); }

Test.cs

using System.Collections.Generic;

static class Test
{
    public static void TestMethod<TEnumerable, TEnumerator, T>(TEnumerable enumerable)
        where TEnumerable : IEnumerable<T, TEnumerator>
        where TEnumerator : IEnumerator<T>
    {
        foreach (var item in enumerable) { }
    }
}