Specific tuples passed to Macro.escape/2 cause it to crash

[kristofka]

Elixir and Erlang/OTP versions

elixir version 1.16. and up, any otp version.

Operating system

any

Current behavior

Passing a three elements tuple where the first element is :quote, the second is anything but a list, and the third is a list with exactly one or two element causes Macro.escape/2 to crash : minimal reproducible example :

iex(1)> Macro.escape({:quote, :foo, [:bar]})
** (FunctionClauseError) no function clause matching in :lists.keydelete3/3

    The following arguments were given to :lists.keydelete3/3:

        # 1
        :column

        # 2
        1

        # 3
        :foo

    (stdlib 5.2.1) lists.erl:885: :lists.keydelete3/3
    (elixir 1.16.2) src/elixir_quote.erl:561: :elixir_quote.meta/2
    (elixir 1.16.2) src/elixir_quote.erl:294: :elixir_quote.do_quote/3
    iex:1: (file)

iex(1)> Macro.escape({:quote, :foo, [:bar, :baz]})
** (FunctionClauseError) no function clause matching in :lists.keydelete3/3

    The following arguments were given to :lists.keydelete3/3:

        # 1
        :column

        # 2
        1

        # 3
        :foo

    (stdlib 5.2.1) lists.erl:885: :lists.keydelete3/3
    (elixir 1.16.2) src/elixir_quote.erl:561: :elixir_quote.meta/2
    (elixir 1.16.2) src/elixir_quote.erl:294: :elixir_quote.do_quote/3
    iex:1: (file)

This issue was first reported in absinthe : #absinthe-graphql/absinthe/issues/1313

Expected behavior

The function should properly escape the tuple :

iex(1)> Macro.escape({:quote, :foo, [:bar]})
{:{}, [], [:quote, :foo, [:bar]]}

A fix is easy enough and I'll submit a PR shortly. However, I'm not entirely convinced that it is possible to completely fix the issue, as tuples of this form may be ambiguous when using Macro.escape/2. For example we have :

iex(1)> Macro.escape({:quote, [column: 10], [:foo]})
{:{}, [], [:quote, [], [:foo]]}
iex(2)> Macro.escape({:quote, [column: 10], [:foo, :bar]})
{:{}, [], [:quote, [], [:foo, :bar]]}
iex(3)> Macro.escape({:quote, [column: 10], [:foo, :bar, :baz]})
{:{}, [], [:quote, [column: 10], [:foo, :bar, :baz]]}
iex(4)>

Which is surprising and arguably a bug as well, and is not solved by the solution I'll submit.

The issue here is that those are caught by the clauses on line 266 of the src/elixir_quote.erl, namely :

do_quote({quote, Meta, [Arg]}, Q) ->
  TArg = do_quote(Arg, Q#elixir_quote{unquote=false}),

  NewMeta = case Q of
    #elixir_quote{op=add_context, context=Context} -> keystore(context, Meta, Context);
    _ -> Meta
  end,

  {'{}', [], [quote, meta(NewMeta, Q), [TArg]]};

do_quote({quote, Meta, [Opts, Arg]}, Q) ->
  TOpts = do_quote(Opts, Q),
  TArg = do_quote(Arg, Q#elixir_quote{unquote=false}),

  NewMeta = case Q of
    #elixir_quote{op=add_context, context=Context} -> keystore(context, Meta, Context);
    _ -> Meta
  end,

  {'{}', [], [quote, meta(NewMeta, Q), [TOpts, TArg]]};

I have attempted but failed to solve the issue at this point, it's unclear to me what careful pattern matching could solve the issue, if any.