"Internal error in sys_core_dsetel" on invalid binary pattern match

[wojtekmach]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.9.0-dev (f56a92202) (compiled with Erlang/OTP 21)

Current behavior

I got a crash on a binary pattern match that's wrong (we can't refer function args like that). I'm not sure if it's worth fixing, but I thought I'll mention this anyway, in case it points to other issues.

defmodule M do
  def decode_int(size, <<value::size(size)-unit(8)>>) do
    value
  end
end

~% elixir a.ex
Function: decode_int/2
** (CompileError) a.ex: internal error in sys_core_dsetel;
crash reason: {case_clause,
    {'EXIT',
        {{case_clause,#{0 => 1,1 => 1}},
         [{sys_core_dsetel,visit_pat,3,
              [{file,"sys_core_dsetel.erl"},{line,239}]},
          {sys_core_dsetel,visit_pats,3,
              [{file,"sys_core_dsetel.erl"},{line,215}]},
          {sys_core_dsetel,visit,2,[{file,"sys_core_dsetel.erl"},{line,212}]},
          {lists,mapfoldl,3,[{file,"lists.erl"},{line,1354}]},
          {sys_core_dsetel,visit,2,[{file,"sys_core_dsetel.erl"},{line,188}]},
          {sys_core_dsetel,visit,2,[{file,"sys_core_dsetel.erl"},{line,126}]},
          {sys_core_dsetel,visit_module_1,3,
              [{file,"sys_core_dsetel.erl"},{line,80}]},
          {sys_core_dsetel,visit_module,1,
              [{file,"sys_core_dsetel.erl"},{line,76}]}]}}}

  in function  compile:'-select_passes/2-anonymous-2-'/3 (compile.erl, line 585)
  in call from compile:'-internal_comp/5-anonymous-1-'/3 (compile.erl, line 349)
  in call from compile:fold_comp/4 (compile.erl, line 376)
  in call from compile:internal_comp/5 (compile.erl, line 360)
  in call from compile:'-do_compile/2-anonymous-0-'/2 (compile.erl, line 177)
  in call from elixir_erl_compiler:compile/4 (src/elixir_erl_compiler.erl, line 52)
  in call from elixir_erl:load_form/5 (src/elixir_erl.erl, line 444)
  in call from elixir_erl_compiler:'-spawn/2-fun-0-'/3 (src/elixir_erl_compiler.erl, line 12)
    (stdlib) lists.erl:1338: :lists.foreach/2
    (elixir) src/elixir_erl_compiler.erl:12: anonymous fn/3 in :elixir_erl_compiler.spawn/2
    (elixir) lib/code.ex:767: Code.require_file/2

FWIW, Erlang does this:

-module(bug).
-export([decode_int/2]).

decode_int(Size, <<Value:Size/unit:8>>) ->
    Value.

===> Compiling src/bug.erl failed
src/bug.erl:4: variable 'Size' is unbound

Expected behavior

Better error message.

[wojtekmach]

Oh, worth mentioning that if I swap function args, it raises a good error message:

  def decode_int(<<value::size(size)-unit(8)>>, size) do
    value
  end

== Compilation error in file lib/foo.ex ==
** (CompileError) lib/foo.ex:60: undefined variable "size"

[josevalim]

Likely applies to v1.8 too.

[fertapric]

I'm currently working on this one :)

[fertapric]

Do we want a custom error for this edge case, or undefined variable "size" would work?

[josevalim]

@fertapric I think we can raise a custom error message here under a certain condition. Here is what I propose:

1. If the variable is not defined prior to the match and not in the match, we raise undefined variable "size"

2. If the variable is not defined prior to the match but it is in the match, we raise:

undefined variable "size" in bitstring segment

If the size of binary is a variable, the variable must be defined prior to the pattern.

Wdyt?

[fertapric]

@josevalim sounds good to me.

If the variable is not defined prior to the match but it is in the match, we raise:

Agree, except for these cases:

<<name_size::size(8), name::binary-size(name_size), " the ", species::binary>> = <<5, "Frank the Walrus">>

[josevalim]

Yes, good catch.