Document map_get behaviour, do not split term types, closes #15209

Author: josevalim

lib/elixir/lib/module/types/apply.ex

@@ -2090,7 +2090,8 @@ defmodule Module.Types.Apply do
   defp args_to_quoted_string(args_types, domain, converter) do
     docs =
       Enum.zip_with(args_types, domain, fn actual, expected ->
-        if compatible?(actual, expected) or not has_simple_difference?(actual, expected) do
+        if compatible?(actual, expected) or not has_simple_difference?(actual, expected) or
+             term_type?(actual) do
           actual |> to_quoted() |> Code.Formatter.to_algebra()
         else
           common = intersection(actual, expected)

lib/elixir/test/elixir/module/types/pattern_test.exs

@@ -726,6 +726,42 @@ defmodule Module.Types.PatternTest do
              """
     end
 
+    test "map_get" do
+      # Below we have a precision warning because the map key is term.
+      # We could solve this in the same we say solved hd/tl in guards,
+      # but we explicitly chose not to (for now) for the following reasons:
+      #
+      # 1. We don't expose `:erlang.map_get` in Elixir, only `map.foo`,
+      #    which is an atom key and will be precise
+      #
+      # 2. The typing violation below will also happen once we add type
+      #    signatures and `%{x => v}` returns anything but a `boolean()`
+      #    (but it will work if you declare the value type only as boolean())
+      #
+      # 3. It also works if you explicitly match on `== true`, which
+      #    effectively forces the return given to `not` to be boolean
+      #
+      # Furthermore, we currently use it to test we don't split term types
+      # in error messages (as we don't have type signatures).
+      assert typeerror!([key, map], not :erlang.map_get(key, map), map) =~ """
+             incompatible types given to Kernel.not/1:
+
+                 not :erlang.map_get(key, map)
+
+             given types:
+
+                 term()
+
+             but expected one of:
+
+                 #1
+                 true
+
+                 #2
+                 false
+             """
+    end
+
     test "when checks" do
       assert typecheck!([x], is_binary(x) when is_atom(x), x) == dynamic(union(binary(), atom()))