Bitstring comprehension result types exclude binaries

[lukaszsamson]

Existing issue

• I have searched existing issues and could not find a duplicate.

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.4.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Interactive Elixir (1.21.0-dev)

Operating system

any

Current behavior

Bitstring comprehension returns union of concatanated types (

elixir/lib/elixir/lib/module/types/expr.ex

Line 639 in 695401e

{opt_union(into_type, intersection), context}

). In case of bitstring fragments that happen to concatanate to full aligned bytes the result falls out of the union type:
bitstring() and not binary() <> bitstring() and not binary() = bitstring()

Repro:

    defmodule Bits do
      def build(items) do
        bits = for _ <- items, into: <<0::4>>, do: <<1::4>>

        case bits do
          x when is_binary(x) -> {:binary, x}
          x -> {:bits, x}
        end
      end
    end

    IO.inspect(Enum.map([[], [1], [1, 2], [1, 2, 3]], &Bits.build/1))

Result:
the code works fine and prints:

[
  bits: <<0::size(4)>>,
  binary: <<1>>,
  bits: <<1, 1::size(4)>>,
  binary: <<1, 17>>
]

Both case branches are needed but the code emits a false positive warning:

warning: the following clause will never match:

    x when is_binary(x) ->

because it attempts to match on the result of:

    bits

which has type:

    bitstring() and not binary()

where "x" was given the type:

    # type: binary()
    # from: iex:26
    is_binary(x)

└─ iex:26: Bits.build/1

Note this issue is distinct from #15465, #15451 and the unaddressed TODO in

elixir/lib/elixir/lib/module/types/expr.ex

Line 626 in 695401e

# TODO: Use the collectable protocol for the output

Expected behavior

Bitstring comprehension result should not be inferred as bitstring() and not binary()