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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

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https://crystal-lang.org/docs/syntax_and_semantics/if_var_nil.html