C# Storing Declaration and Type Pattern Result Causes CS0165 (Use of Unassigned Local Variable)

I'm curious why both of the following code segments produces error CS0165: Use of unassigned local variable 'i':

// Attempt #1
object obj = 4;
bool isInt = obj is int i;
if (isInt)
    Console.WriteLine("obj = {0}", i); // CS0165

// Attempt #2
object obj = 4;
bool isInt;
if (isInt = obj is int i)
    Console.WriteLine("obj = {0}", i); // CS0165

The following compiles, but seems like it shouldn't be necessary:

object obj = 4;
bool isInt = false;
if (obj is int i)
{
    isInt = true;
    Console.WriteLine("obj = {0}", i);
}

Is there a less verbose way to use C# Declaration and Type Patterns when attempting to store the result of the test condition?