Syntax of isExpression

[Alex via Digitalmars-d-learn]

Hey guys,
could you help me understand the syntax of the isExpression?
I have an example, leaned on documentation

https://dlang.org/spec/expression.html#IsExpression

case 7.

// Code starts here //
import std.stdio, std.typecons;

alias Tup = Tuple!(int, string, bool);

enum myType {a, b, c}

struct E(T, myType ET, bool P) { T parts; }

void main()
{
     auto r = E!(int, myType.a, true)();
     fun(Tup.init);
     writeln("-----");
     fun(r);
}

void fun(T)(T t)
{
     static if (is(T : TX!TL, alias TX, TL...))
     {
         writeln(is(TL[0] == int));
         writeln(typeid(TL[1]));
         writeln(typeid(TL[2]));
         writeln(is(TL[2] == bool));
     }
}
// Code ends here //

The question is: why during testing with Tup the last line gives 
"true" and with the struct it is false? Especially, while the 
line before is the same for both cases.