`map_fetch_key` result is representation dependant

[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

Let's create equal types t and t2 by identity t2 = cover \ (cover \ t) with cover = t u knife

Repro:

t = closed_map([a: term()])
knife = open_map([c: none()])

cover = opt_union(t, knife)
t2 = opt_difference(cover, opt_difference(cover, t))

equal?(t, t2) # true

map_fetch_key(t, :a) # {false, :term}, correct
map_fetch_key(t2, :a) # :badkey, wrong

t representation

%{map: {-97320740, :closed, [a: :term]}}

t2 representation

%{
  map: {2486087065, {-97320740, :closed, [a: :term]}, :bdd_top,
   {105403943, :open, [c: %{}]}, :bdd_bot}
}

Related to #15491

Expected behavior

Result should be the same regardless of representation