Return type of Map.pop is too narrow

Existing issue

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

Elixir and Erlang/OTP versions

Erlang/OTP 29 [erts-17.0.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.20.1 (compiled with Erlang/OTP 29)

Operating system

MacOS Tahoe 26.1

Current behavior

defmodule Repro do
 # pop an element from a non-empty map
  def pop(map, key) when map_size(map) > 0 do
    {_value, rest} = Map.pop!(map, key)
    rest == %{}
  end
  
  def test do
    map = %{"some_key" => 1}
    pop(map, "some_key")
  end
end

Generates the following compiler output:

    warning: comparison between distinct types found:

        rest == %{}

    given types:

        dynamic(map() and not empty_map()) == empty_map()

    where "rest" was given the type:

        # type: dynamic(map() and not empty_map())
        # from: bug.ex:3:20
        {_value, rest} = Map.pop!(map, key)

    While Elixir can compare across all types, you are comparing across types which are always disjoint, and the result is either always true or always false

    type warning found at:
    │
  4 │     rest == %{}
    │          ~
    │
    └─ bug.ex:4:10: Repro.pop/2

Expected behavior

The compiler correctly marks the type of the map value as dynamic(map() and not empty_map()), but after calling Map.pop!/2 the type of rest can be empty_map(), so I think the right type is supposed to be just map().

I have found this PR: #15326, but I do not think they are the same. This one is for a tautological guard (checking a map size is bigger or equal to 0), and this one is not.

These are more isolated tests:

diff --git a/lib/elixir/test/elixir/module/types/descr_test.exs b/lib/elixir/test/elixir/module/types/descr_test.exs
index b48049f2c..12b074eca 100644
--- a/lib/elixir/test/elixir/module/types/descr_test.exs
+++ b/lib/elixir/test/elixir/module/types/descr_test.exs
@@ -2712,6 +2712,13 @@ test "with non-empty open maps does not call the callback with none from absent
       refute_received :callback_invoked
     end

+    test "deleting domain keys from non-empty maps may return an empty map" do
+      type = dynamic(opt_difference(open_map(), empty_map()))
+
+      assert {_value, descr, []} = map_update(type, binary(), not_set(), true, false)
+      assert subtype?(empty_map(), descr)
+    end
+
     test "with dynamic atom keys" do
       assert {type, descr, errors} =
                map_update(closed_map(key: atom([:value])), dynamic(), atom([:new_value]))
diff --git a/lib/elixir/test/elixir/module/types/map_test.exs b/lib/elixir/test/elixir/module/types/map_test.exs
index d871af0a9..1e206673a 100644
--- a/lib/elixir/test/elixir/module/types/map_test.exs
+++ b/lib/elixir/test/elixir/module/types/map_test.exs
@@ -804,6 +804,15 @@ test "checking" do

       assert typecheck!([x], Map.pop!(x, 123)) ==
                dynamic(tuple([term(), open_map()]))
+
+      assert typecheck!(
+               [x],
+               map_size(x) > 0,
+               (
+                 {_value, rest} = Map.pop!(x, "some_key")
+                 rest
+               )
+             ) == dynamic(open_map())
     end

     test "inference" do

Fix map domain update

Sunbelt Computer Software

PL/B Language Development and Support

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Return type of Map.pop is too narrow #15482

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Expected behavior

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Sunbelt Computer Software

PL/B Language Development and Support

Return type of Map.pop is too narrow #15482

Description

Existing issue

Elixir and Erlang/OTP versions

Operating system

Current behavior

Expected behavior

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josevalim commented on Jun 15, 2026

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