Why does this compile?

private fun shouldNotCompile(optionalString: String?): Boolean {
        optionalString?.let {
            return true
        }
    }

I’ve completely forgotten to specify a return value for the case where the

let

is not entered.

Weeeell, it might be considered a bug, but... Compiler optimizes it to

boolean shouldNotCompile(@Nullable String optionalString) {
  return optionalString != null
}

Probably expected behavior due to optimizations.

Is what appears to be happening, but why would it do that though?

No, it's a bug.

fun shouldNotCompile(optionalString: String?): Boolean {
    optionalString?.let {
        println("something")
        return true
    }
}

Decompiles to

boolean shouldNotCompile(@Nullable String optionalString) {
      if (optionalString != null) {
         String var3 = "something";
         System.out.println(var3);
         return true;
      } else {
         return false;
      }
   }

Which sure should not happen.

Yes, it's a bug related to contracts and control flow analysis https://youtrack.jetbrains.com/issue/KT-28061