Elixir: Invalid unused variable warning

[febeling]

Environment

• Elixir & Erlang versions (elixir --version):
  1.6.1
• Operating system:
  macos 10.13.3 (17D102)

Current behavior

This test generates warnings about unused variable found.

defmodule UnusedTest do
  use ExUnit.Case

  def fun, do: %{}

  test "unused warning repro" do
    found = fun()
    assert match?(found, %{})
  end
end

warning: variable "found" is unused
  test/projections/unused_test.exs:9

warning: variable "found" is unused
  test/projections/unused_test.exs:10

Expected behavior

No warning, as the variable is actually used in assert's match.

[michalmuskala]

The warning is correct. The first argument of match? is a pattern. A variable used in pattern matches any value. Your test is exactly the same as:

  test "unused warning repro" do
    found = fun()
    assert match?(x, %{})
  end

Most probably you wanted:

  test "unused warning repro" do
    found = fun()
    assert match?(^found, %{})
  end

[febeling]

Gotcha. Makes sense. Thanks!