Quick Answer

Compiler tracks type checks and auto-casts

Understanding the Issue

After performing an is-check, Kotlin automatically casts the variable to that type within the scope of the check, eliminating the need for explicit casting.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Explicit casting
if (obj is String) {
    val length = (obj as String).length // Redundant cast
}

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Smart cast
if (obj is String) {
    val length = obj.length // Automatically cast to String
}

Key Takeaways

Let the compiler handle safe casting after type checks.