Mutable variables - let 1 become 0

[michalmuskala]

Describe the bug
In some specific cases pattern matching doesn't do what it's supposed to - instead it mutates variables.

To Reproduce
Given the following snippet:

-module(test).

-export([test/0]).

test() ->
    Zero = 0,
    One = 1,

    Result = One = Zero,

    io:format("~p ~p~n", [Zero, One]),
    Result.

Compiling the test module and running test:test() from the shell produces:

1> test:test().
0 0
0

Expected behavior
The code should crash with match error, and variables should remain immutable.

Affected versions
Tested with the latest maint-25 and maint-24 branches. This correctly fails on current master, even emitting a warning during compilation with erlc.

Additional context
Originally discovered by @jcpetruzza

[michalmuskala]

This also incorrectly succeeds and rebinds a variable with other expressions in the match tail, not just plain variables, e.g.

test2() ->
    Zero = 0,

    Result = Zero = self(),
    io:format("~p~n", [Zero]),
    Result.

1> test:test2().
<0.81.0>
<0.81.0>

[bjorng]

It is also incorrect in OTP 23 and probably in many earlier releases.

It seems that the bug was inadvertently corrected in #6415. My current plan is to backport part of that PR to OTP 23 through 25 in order to fix this bug.

[michalmuskala]

Looks like the comment I was replying to is gone now

fun arguments allow shadowing, if you mean code similar to the example below, it's working as documented.

test3() ->
    Zero = 0,
    One = 1,

    F = fun(One) -> io:format("~p~n", [One]) end,
    F(Zero).

The compiler also issues a warning about this:

test.erl:23:13: Warning: variable 'One' shadowed in 'fun'
%   23|     F = fun(One) -> io:format("~p~n", [One]) end,
%     |             ^

[michalmuskala]

Thanks for fixing this quickly!