Miscompilation with binary pattern matching and select_val

[michalmuskala]

Describe the bug
Given the following code:

-module(test).

-export([test/1]).

-define(is_valid(B), B =:= 1; B =:= 2).
-define(is_not_valid(B), B =:= 0; B =:= 3).

test(Bin) when is_binary(Bin) -> test1(Bin, 0).

test1(<<B1, B2, Rest/binary>>, N) when ?is_valid(B1), ?is_valid(B2) -> 
    test1(Rest, N + 2);
test1(<<B, Rest/binary>>, N) when ?is_valid(B) ->
    test1(Rest, N + 1);
test1(<<B, _Rest/binary>>, N) when ?is_not_valid(B) ->
    error(N);
test1(_, N) -> N.

The code is not behaving as expected when called as:

1> test:test(<<1, 1, 1, 1, 1>>).
5
2> test:test(<<1, 0, 1, 1, 1>>).
5
3>

The second call, that includes the number 0 should clearly trigger the error clause and result in an exception.

Importantly, removing the multi-byte case (first clause) results in correct behaviour.

Affected versions
Recent master

Additional context
This was discovered while working on further optimising #8111

[michalmuskala]

False alarm - turns out macros, ;, and , have precedence implications I didn't foresee.
This was parsed as when (B1 =:= 1); (B1 =:= 2, B2 =:= 1); (B2 =:= 2) instead of
when (B1 =:= 1; B1 =:= 2), (B2 =:= 1; B2 =:= 2) that I expected.