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