scoping compiler bug on cond

[vans163]

Environment

OTP 21.2
Elixir 1.7.3

defmodule Test do
    def handle_ms(ms) do
        Map.put(ms, 5, 5)
    end

    def test() do
        ms = %{}
        ms_new = cond do
            true ->
                ms = Map.put(ms, :wtf, 5)
                handle_ms(ms)
        end
        {ms, ms_new}
    end
end

1> Test.test
{%{wtf: 5}, %{5 => 5, :wtf => 5}}

Its clear the evaluation of this is incorrect and should be {%{}, %{5 => 5, :wtf => 5}}

[michalmuskala]

It looks like this is a bug in the compiler related to "flattening" of a cond with just one branch. A cond with two branches is compiled correctly.

[vans163]

I fibbed a little, the bug is present in this version of the runtime (that was running on the machine).

Erlang/OTP 21 [erts-10.0] [source] [64-bit] [smp:40:40] [ds:40:40:10] [async-threads:1] [hipe]
Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

Its fixed in the later release of Elixir which I used as a test environment, BUT that single branch case is still buggy. The version 1.6.6 had it so multibranch exhibited the same bug.

[michalmuskala]

In versions before 1.7 the behaviour you saw would be expected. It was a long-standing scoping compatibility with pre-1.0 code. This code would compile, but would issue a warning. The compatibility was dropped in 1.7, because otherwise we wouldn't be able to fix some other bugs in the compiler.

[josevalim]

Closing in favor of #8544.