The pin doesn't pin

[michalmuskala]

Elixir and Erlang/OTP versions

Erlang/OTP 25 [erts-13.1.4] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]

Elixir 1.14.3 (compiled with Erlang/OTP 25)

Operating system

macOS

Current behavior

Given the following module:

defmodule Test do
  def test() do
    zero = 0
    one = 1

    result = ^one = zero
    IO.inspect({zero, one})
    result
  end
end

Running Test.test() outputs:

iex(1)> Test.test()
{0, 0}
0

Expected behavior

The code should fail with a match error.

This is caused by an issue in Erlang: erlang/otp#6873. The issue exists at least in OTP 24 and 25. The current OTP master does not present it.
However, given how serious in terms of language semantics the issue is, I decided to report it here as well - Elixir might want to employ some sort of mitigation through compiling to a different code pattern.

[josevalim]

Thank you!

[sopotc]

The issue exists at least in OTP 24 and 25

Also in 23 as confirmed by OTP.

[josevalim]

Closing as it has been backported to OTP 23, 24, 25. Thanks @michalmuskala !