a = 23
if(true, do: a = 4)

warning: variable "a" is unused

Note variables defined inside case, cond, fn, if and similar do not leak. If you want to conditionally override an existing variable "a", you will have to explicitly return the variable. For example:

    if some_condition? do
      atom = :one
    else
      atom = :two
    end

should be written as

    atom =
      if some_condition? do
        :one
      else
        :two
      end

Unused variable found at:

elixir/CHANGELOG.md at v1.7 · elixir-lang/elixir · GitHub

[Kernel] Raise on unsafe variables in order to allow us to better track unused variable

Release v1.7.0 · elixir-lang/elixir · GitHub

[Kernel] Raise on unsafe variables in order to allow us to better track unused variables (also known as imperative assignment / variable leakage)