Determine if the language is $R$ Consider the following language: 

$$L = \{ \langle M \rangle \ |\ M \text { is a TM that decides the halting problem} \}$$


  determine whether or not the language is in $R$.


Now, from my understanding an $\langle M \rangle \in L$ doesn't necessarily returns the right answer but rather halts for every $\langle P, x \rangle$ where $p$ is a program (=TM) and $x$ is an input for the program.   

I am guessing the language isn't decidable and can be showed as such by some reduction, but couldn't think of something useful. 

I'll be glad for help.
comments summary:

The language $L$ is decidable.

Hint:


   $L$ is in fact empty! It contains all the machines that decide the halting problem. But, the halting problem is undecidable => there are no machines that decide it.