Incorrect expansion of `<<?a>> <> rest`

[alco]

Macro.to_string Macro.expand_all quote(do: <<?a>> <> rest = "abc"), __ENV__
"<<<<97>> :: binary, rest :: binary>> = \"abc\""

This is an invalid pattern. I was expecting it to produce either

<< <<97>>, rest :: binary>> = "abc"

or

<< 97, rest :: binary>> = "abc"

both of which match.

Compare to this

Macro.to_string Macro.expand_all quote(do: "a" <> rest = "abc"), __ENV__
"<<\"a\", rest :: binary>> = \"abc\""