What exactly is a “Special Class”?

Question

After failing to get something like the following to compile:

public class Gen where T : System.Array
{
}

with the error

Answer 1

From the Roslyn source code, it looks like a list of hardcoded types:

switch (type.SpecialType)
{
    case SpecialType.System_Object:
    case SpecialType.System_ValueType:
    case SpecialType.System_Enum:
    case SpecialType.System_Delegate:
    case SpecialType.System_MulticastDelegate:
    case SpecialType.System_Array:
        // "Constraint cannot be special class '{0}'"
        Error(diagnostics, ErrorCode.ERR_SpecialTypeAsBound, syntax, type);
        return false;
}

Source: Binder_Constraints.cs IsValidConstraintType
I've found it using a GitHub search: "A constraint cannot be special class"