[T(Index) Is Nothing] is [CObj(T(Index)) Is Nothing] instead [T(Index) Is Default].

```vb
Public Function empty_index(Of T)(Host As T()) As Long
Dim Length = Host.LongLength,
Pos = 0L
Do
If Pos >= Length Then Return -1

'Not work with value type.
If Host(Pos) Is Nothing Then Return Pos

'Because it's compile to this.
If CObj(Host(Pos)) Is Nothing Then Return Pos

'While this work well with value type and reference type.
If Object.Equals(Input, CType(Nothing, T)) Then Return Pos

Pos += 1
Loop
End Function
```

## Why not Object.Equals ?
Simple answer is super cut corner for performance as you see in IL code below.

```
ldarg.0
ldloc.2
conv.i4
ldelem !!T
box !!T
// call bool [System.Runtime]System.Object::Equals(object,object)
brtrue.s IL_0023
```

Instead call `Object.Equals` it use `0` aka `null pointer` compare result so when `brtrue` which is branch on true and true in this case is anything except `0`.

## How to fixed it ?

Just reverse back to use `Object.Equals`.
```vb

Public Function is_default(Of T)(Input As T) As Boolean
Return Object.Equals(Input, CType(Nothing, T))
End Function
```

Or double down it, cut more corner.
```
.method public static bool is_default(!!T Input) cil managed
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.ExtensionAttribute::.ctor() = ( 01 00 00 00 )
.maxstack 2

ldarg.0
ldc.i4.0
ceq
ret
}
```