if var.nil?

If an if's condition is var.nil? then the type of var in the then branch is known by the compiler to be Nil, and to be known as non-Nil in the else branch:

a = some_condition ? nil : 3
if a.nil?
  # here a is Nil
else
  # here a is Int32
end

Instance Variables

Type restriction through if var.nil? only occurs with local variables. The type of an instance variable in a similar code example to the one above will still be nilable and will throw a compile error since greet expects a String in the unless branch.

class Person
  property name : String?

  def greet
    unless @name.nil?
      puts "Hello, #{@name.upcase}" # Error: undefined method 'upcase' for Nil (compile-time type is (String | Nil))
    else
      puts "Hello"
    end
  end
end

Person.new.greet

You can solve this by storing the value in a local variable first:

def greet
  name = @name
  unless name.nil?
    puts "Hello, #{name.upcase}" # name will be String - no compile error
  else
    puts "Hello"
  end
end

This is a byproduct of multi-threading in Crystal. Due to the existence of Fibers, Crystal does not know at compile-time whether the instance variable will still be non-Nil when the usage in the if branch is reached.