Introduce is_map_key/2 guard BIF

This complements the `map_get/2` guard BIF introduced in #1784.

Rationale.

`map_get/2` allows accessing map fields in guards, but it might be
problematic in more complex guard expressions, for example:

foo(X) when map_get(a, X) =:= 1 or is_list(X) -> ...

The `is_list/1` part of the guard could never succeed since the
`map_get/2` guard would fail the whole guard expression. In this
situation, this could be solved by using `;` instead of `or` to separate
the guards, but it is not possible in every case.

To solve this situation, this PR proposes a `is_map_key/2` guard that
allows to check if a map has key inside a guard before trying to access
that key. When combined with `is_map/1` this allows to construct a
purely boolean guard expression testing a value of a key in a map.

Implementation.

Given the use case motivating the introduction of this function, the PR
contains compiler optimisations that produce optimial code for the
following guard expression:

foo(X) when is_map(X) and is_map_key(a, X) and map_get(a, X) =:= 1 -> ok;
foo(_) -> error.

Given all three tests share the failure label, the `is_map_key/2` and
`is_map/2` tests are optimised away.

As with `map_get/2` the `is_map_key/2` BIF is allowed in match specs.

18 files changed, 165 insertions, 15 deletions

diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index f561413fab..3154fdaf8c 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -2432,6 +2432,26 @@ os_prompt%</pre>
     </func>
 
     <func>
+      <name name="is_map_key" arity="2"/>
+      <fsummary></fsummary>
+      <desc>
+        <p>Returns <c>true</c> if map <c><anno>Map</anno></c> contains
+          <c><anno>Key</anno></c> and returns <c>false</c> if it does not
+          contain the <c><anno>Key</anno></c>.</p>
+        <p>The call fails with a <c>{badmap,Map}</c> exception if
+          <c><anno>Map</anno></c> is not a map.</p>
+        <p><em>Example:</em></p>
+        <code type="none">
+> Map = #{"42" => value}.
+#{"42" => value}
+> is_map_key("42",Map).
+true
+> is_map_key(value,Map).
+false</code>
+      </desc>
+    </func>
+
+    <func>
       <name name="is_number" arity="1"/>
       <fsummary>Check whether a term is a number.</fsummary>
       <desc>
diff --git a/erts/doc/src/match_spec.xml b/erts/doc/src/match_spec.xml
index 888366b239..46a3daebe8 100644
--- a/erts/doc/src/match_spec.xml
+++ b/erts/doc/src/match_spec.xml
@@ -86,12 +86,12 @@
         <c><![CDATA[is_list]]></c> | <c><![CDATA[is_number]]></c> |
         <c><![CDATA[is_pid]]></c> | <c><![CDATA[is_port]]></c> |
         <c><![CDATA[is_reference]]></c> | <c><![CDATA[is_tuple]]></c> |
-        <c><![CDATA[is_map]]></c> | <c><![CDATA[is_binary]]></c> |
-        <c><![CDATA[is_function]]></c> | <c><![CDATA[is_record]]></c> |
-        <c><![CDATA[is_seq_trace]]></c> | <c><![CDATA['and']]></c> |
-        <c><![CDATA['or']]></c> | <c><![CDATA['not']]></c> |
-        <c><![CDATA['xor']]></c> | <c><![CDATA['andalso']]></c> |
-        <c><![CDATA['orelse']]></c>
+        <c><![CDATA[is_map]]></c> | <c><![CDATA[is_map_key]]></c> |
+        <c><![CDATA[is_binary]]></c> | <c><![CDATA[is_function]]></c> |
+        <c><![CDATA[is_record]]></c> | <c><![CDATA[is_seq_trace]]></c> |
+        <c><![CDATA['and']]></c> | <c><![CDATA['or']]></c> |
+        <c><![CDATA['not']]></c> | <c><![CDATA['xor']]></c> |
+        <c><![CDATA['andalso']]></c> | <c><![CDATA['orelse']]></c>
       </item>
       <item>ConditionExpression ::= ExprMatchVariable | { GuardFunction } |
         { GuardFunction, ConditionExpression, ... } | TermConstruct
@@ -168,11 +168,12 @@
         <c><![CDATA[is_list]]></c> | <c><![CDATA[is_number]]></c> |
         <c><![CDATA[is_pid]]></c> | <c><![CDATA[is_port]]></c> |
         <c><![CDATA[is_reference]]></c> | <c><![CDATA[is_tuple]]></c> |
-        <c><![CDATA[is_map]]></c> | <c><![CDATA[is_binary]]></c> |
-        <c><![CDATA[is_function]]></c> | <c><![CDATA[is_record]]></c> |
-        <c><![CDATA['and']]></c> | <c><![CDATA['or']]></c> |
-        <c><![CDATA['not']]></c> | <c><![CDATA['xor']]></c> |
-        <c><![CDATA['andalso']]></c> | <c><![CDATA['orelse']]></c>
+        <c><![CDATA[is_map]]></c> | <c><![CDATA[map_is_key]]></c> |
+        <c><![CDATA[is_binary]]></c> | <c><![CDATA[is_function]]></c> |
+        <c><![CDATA[is_record]]></c> | <c><![CDATA['and']]></c> |
+        <c><![CDATA['or']]></c> | <c><![CDATA['not']]></c> |
+        <c><![CDATA['xor']]></c> | <c><![CDATA['andalso']]></c> |
+        <c><![CDATA['orelse']]></c>
       </item>
       <item>ConditionExpression ::= ExprMatchVariable | { GuardFunction } |
         { GuardFunction, ConditionExpression, ... } | TermConstruct
diff --git a/erts/emulator/beam/bif.tab b/erts/emulator/beam/bif.tab
index 276bef2bbb..33738dc20b 100644
--- a/erts/emulator/beam/bif.tab
+++ b/erts/emulator/beam/bif.tab
@@ -696,3 +696,4 @@ bif ets:whereis/1
 bif erts_internal:gather_alloc_histograms/1
 bif erts_internal:gather_carrier_info/1
 ubif erlang:map_get/2
+ubif erlang:is_map_key/2
diff --git a/erts/emulator/beam/erl_db_util.c b/erts/emulator/beam/erl_db_util.c
index 6354abfd1f..ef22cda1f0 100644
--- a/erts/emulator/beam/erl_db_util.c
+++ b/erts/emulator/beam/erl_db_util.c
@@ -644,6 +644,12 @@ static DMCGuardBif guard_tab[] =
         DBIF_ALL
     },
     {
+        am_is_map_key,
+        &is_map_key_2,
+        2,
+        DBIF_ALL
+    },
+    {
 	am_bit_size,
 	&bit_size_1,
 	1,
diff --git a/erts/emulator/beam/erl_map.c b/erts/emulator/beam/erl_map.c
index f577b017c3..3d565b1bb8 100644
--- a/erts/emulator/beam/erl_map.c
+++ b/erts/emulator/beam/erl_map.c
@@ -43,6 +43,7 @@
  *
  * DONE:
  * - erlang:is_map/1
+ * - erlang:is_map_key/2
  * - erlang:map_size/1
  * - erlang:map_get/2
  *
@@ -919,7 +920,7 @@ static int hxnodecmp(hxnode_t *a, hxnode_t *b) {
 	return -1;
 }
 
-/* maps:is_key/2 */
+/* maps:is_key/2 and erlang:is_map_key/2 */
 
 BIF_RETTYPE maps_is_key_2(BIF_ALIST_2) {
     if (is_map(BIF_ARG_2)) {
@@ -929,6 +930,10 @@ BIF_RETTYPE maps_is_key_2(BIF_ALIST_2) {
     BIF_ERROR(BIF_P, BADMAP);
 }
 
+BIF_RETTYPE is_map_key_2(BIF_ALIST_2) {
+    BIF_RET(maps_is_key_2(BIF_CALL_ARGS));
+}
+
 /* maps:keys/1 */
 
 BIF_RETTYPE maps_keys_1(BIF_ALIST_1) {
diff --git a/erts/emulator/test/map_SUITE.erl b/erts/emulator/test/map_SUITE.erl
index 43807b4388..f93c637650 100644
--- a/erts/emulator/test/map_SUITE.erl
+++ b/erts/emulator/test/map_SUITE.erl
@@ -38,6 +38,7 @@
          t_map_size/1,
          t_map_get/1,
          t_is_map/1,
+         t_is_map_key/1,
 
          %% Specific Map BIFs
          t_bif_map_get/1,
@@ -718,7 +719,47 @@ t_map_get(Config) when is_list(Config) ->
     true = if map_get(a, M2) =:= 1 -> true; true -> false end,
     false = if map_get(x, M2) =:= 1 -> true; true -> false end,
     do_badmap(fun
-        (T) when map_get(T, x) =:= 1 -> ok;
+        (T) when map_get(x, T) =:= 1 -> ok;
+        (T) -> false = is_map(T)
+    end),
+    ok.
+
+t_is_map_key(Config) when is_list(Config) ->
+    %% small map
+    true = is_map_key(a, id(#{a=>1})),
+    true = is_map_key(b, id(#{a=>1, b=>2})),
+    true = is_map_key("hello", id(#{a=>1, "hello"=>"hi"})),
+    true = is_map_key({1,1.0}, id(#{a=>a, {1,1.0}=>"tuple hi"})),
+
+    M0   = id(#{ k1=>"v1", <<"k2">> => <<"v3">> }),
+    true = is_map_key(<<"k2">>, M0#{<<"k2">> => "v4"}),
+
+    %% large map
+    M1   = maps:from_list([{I,I}||I<-lists:seq(1,100)] ++
+			  [{a,1},{b,2},{"hello","hi"},{{1,1.0},"tuple hi"},
+			   {k1,"v1"},{<<"k2">>,"v3"}]),
+    true = is_map_key(a, M1),
+    true = is_map_key(b, M1),
+    true = is_map_key("hello", M1),
+    true = is_map_key({1,1.0}, M1),
+    true = is_map_key(<<"k2">>, M1),
+
+    %% error cases
+    do_badmap(fun(T) ->
+		      {'EXIT',{{badmap,T},[{erlang,is_map_key,_,_}|_]}} =
+			  (catch is_map_key(a, T))
+	      end),
+
+    false = is_map_key({1,1}, id(#{{1,1.0}=>"tuple"})),
+    false = is_map_key(a, id(#{})),
+    false = is_map_key(a, id(#{b=>1, c=>2})),
+
+    %% in guards
+    M2 = id(#{a=>1}),
+    true = if is_map_key(a, M2) -> true; true -> false end,
+    false = if is_map_key(x, M2) -> true; true -> false end,
+    do_badmap(fun
+        (T) when is_map_key(T, x) =:= 1 -> ok;
         (T) -> false = is_map(T)
     end),
     ok.
diff --git a/erts/emulator/test/match_spec_SUITE.erl b/erts/emulator/test/match_spec_SUITE.erl
index c1bc01f01e..4415d8d1b9 100644
--- a/erts/emulator/test/match_spec_SUITE.erl
+++ b/erts/emulator/test/match_spec_SUITE.erl
@@ -898,6 +898,13 @@ maps(Config) when is_list(Config) ->
     {ok,false,[],[]} = erlang:match_spec_test(not_a_map, [{'$1',[{map_get,b,'$1'}],['$_']}], table),
     {ok,true,[],[]} = erlang:match_spec_test(#{a => true}, [{'$1',[{map_get,a,'$1'}],[true]}], table),
 
+    {ok,true,[],[]} = erlang:match_spec_test(#{a => 1}, [{'$1',[],[{is_map_key,a,'$1'}]}], table),
+    {ok,false,[],[]} = erlang:match_spec_test(#{a => 1}, [{'$1',[],[{is_map_key,b,'$1'}]}], table),
+    {ok,'EXIT',[],[]} = erlang:match_spec_test(not_a_map, [{'$1',[],[{is_map_key,a,'$1'}]}], table),
+    {ok,false,[],[]} = erlang:match_spec_test(#{a => 1}, [{'$1',[{is_map_key,b,'$1'}],['$_']}], table),
+    {ok,false,[],[]} = erlang:match_spec_test(not_a_map, [{'$1',[{is_map_key,b,'$1'}],['$_']}], table),
+    {ok,true,[],[]} = erlang:match_spec_test(#{a => true}, [{'$1',[{is_map_key,a,'$1'}],[true]}], table),
+
     %% large maps
 
     Ls0 = [{I,<<I:32>>}||I <- lists:seq(1,415)],
diff --git a/erts/preloaded/src/erlang.erl b/erts/preloaded/src/erlang.erl
index 53e90a4f2d..5fcad25c6d 100644
--- a/erts/preloaded/src/erlang.erl
+++ b/erts/preloaded/src/erlang.erl
@@ -135,7 +135,7 @@
 -export([insert_element/3]).
 -export([integer_to_binary/1, integer_to_list/1]).
 -export([iolist_size/1, iolist_to_binary/1, iolist_to_iovec/1]).
--export([is_alive/0, is_builtin/3, is_process_alive/1, length/1, link/1]).
+-export([is_alive/0, is_builtin/3, is_map_key/2, is_process_alive/1, length/1, link/1]).
 -export([list_to_atom/1, list_to_binary/1]).
 -export([list_to_bitstring/1, list_to_existing_atom/1, list_to_float/1]).
 -export([list_to_integer/1, list_to_integer/2]).
@@ -1121,6 +1121,13 @@ is_alive() ->
 is_builtin(_Module, _Function, _Arity) ->
     erlang:nif_error(undefined).
 
+%% Shadowed by erl_bif_types: erlang:is_map_key/2
+-spec is_map_key(Key, Map) -> boolean() when
+    Key :: term(),
+    Map :: map().
+is_map_key(_,_) ->
+    erlang:nif_error(undef).
+
 %% is_process_alive/1
 -spec is_process_alive(Pid) -> boolean() when
       Pid :: pid().
diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index dbbaae05eb..762c7bdf9e 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -392,6 +392,26 @@ backward([{bif,'or',{f,To0},[Dst,{atom,false}],Dst}=I|Is], D,
 	_ ->
 	    backward(Is, D, [I|Acc])
     end;
+backward([{bif,map_get,{f,FF},[Key,Map],_}=I0,
+          {test,has_map_fields,{f,FT}=F,[Map|Keys0]}=I1|Is], D, Acc) when FF =/= 0 ->
+    case shortcut_label(FF, D) of
+        FT ->
+            case lists:delete(Key, Keys0) of
+                [] ->
+                    backward([I0|Is], D, Acc);
+                Keys ->
+                    Test = {test,has_map_fields,F,[Map|Keys]},
+                    backward([Test|Is], D, [I0|Acc])
+            end;
+        _ ->
+            backward([I1|Is], D, [I0|Acc])
+    end;
+backward([{bif,map_get,{f,FF},[_,Map],_}=I0,
+          {test,is_map,{f,FT},[Map]}=I1|Is], D, Acc) when FF =/= 0 ->
+    case shortcut_label(FF, D) of
+        FT -> backward([I0|Is], D, Acc);
+        _ -> backward([I1|Is], D, [I0|Acc])
+    end;
 backward([I|Is], D, Acc) ->
     backward(Is, D, [I|Acc]);
 backward([], _D, Acc) -> Acc.
diff --git a/lib/compiler/src/beam_peep.erl b/lib/compiler/src/beam_peep.erl
index eb3192fe8f..920fb00397 100644
--- a/lib/compiler/src/beam_peep.erl
+++ b/lib/compiler/src/beam_peep.erl
@@ -77,6 +77,12 @@ peep([{bif,tuple_size,_,[_]=Ops,Dst}=I|Is], SeenTests0, Acc) ->
     %% Kill all remembered tests that depend on the destination register.
     SeenTests = kill_seen(Dst, SeenTests1),
     peep(Is, SeenTests, [I|Acc]);
+peep([{bif,map_get,_,[Key,Map],Dst}=I|Is], SeenTests0, Acc) ->
+    %% Pretend that we have seen {test,has_map_fields,_,[Map,Key]}
+    SeenTests1 = gb_sets:add({has_map_fields,[Map,Key]}, SeenTests0),
+    %% Kill all remembered tests that depend on the destination register.
+    SeenTests = kill_seen(Dst, SeenTests1),
+    peep(Is, SeenTests, [I|Acc]);
 peep([{bif,_,_,_,Dst}=I|Is], SeenTests0, Acc) ->
     %% Kill all remembered tests that depend on the destination register.
     SeenTests = kill_seen(Dst, SeenTests0),
diff --git a/lib/compiler/src/beam_type.erl b/lib/compiler/src/beam_type.erl
index 28f36db399..12da8c9446 100644
--- a/lib/compiler/src/beam_type.erl
+++ b/lib/compiler/src/beam_type.erl
@@ -462,6 +462,9 @@ update({set,[D],[Index,Reg],{bif,element,_}}, Ts0) ->
               end,
     Ts = tdb_meet(Reg, {tuple,min_size,MinSize,[]}, Ts0),
     tdb_store(D, any, Ts);
+update({set,[D],[_Key,Map],{bif,map_get,_}}, Ts0) ->
+    Ts = tdb_meet(Map, map, Ts0),
+    tdb_store(D, any, Ts);
 update({set,[D],Args,{bif,N,_}}, Ts) ->
     Ar = length(Args),
     BoolOp = erl_internal:new_type_test(N, Ar) orelse
diff --git a/lib/compiler/src/erl_bifs.erl b/lib/compiler/src/erl_bifs.erl
index 70b36f029e..a7452aebc8 100644
--- a/lib/compiler/src/erl_bifs.erl
+++ b/lib/compiler/src/erl_bifs.erl
@@ -94,6 +94,7 @@ is_pure(erlang, is_function, 1) -> true;
 is_pure(erlang, is_integer, 1) -> true;
 is_pure(erlang, is_list, 1) -> true;
 is_pure(erlang, is_map, 1) -> true;
+is_pure(erlang, is_map_key, 2) -> true;
 is_pure(erlang, is_number, 1) -> true;
 is_pure(erlang, is_pid, 1) -> true;
 is_pure(erlang, is_port, 1) -> true;
diff --git a/lib/compiler/src/v3_codegen.erl b/lib/compiler/src/v3_codegen.erl
index 8e73b613a0..9652a8476d 100644
--- a/lib/compiler/src/v3_codegen.erl
+++ b/lib/compiler/src/v3_codegen.erl
@@ -589,6 +589,7 @@ is_gc_bif(element, 2) -> false;
 is_gc_bif(get, 1) -> false;
 is_gc_bif(tuple_size, 1) -> false;
 is_gc_bif(map_get, 2) -> false;
+is_gc_bif(is_map_key, 2) -> false;
 is_gc_bif(Bif, Arity) ->
     not (erl_internal:bool_op(Bif, Arity) orelse
 	 erl_internal:new_type_test(Bif, Arity) orelse
@@ -1620,6 +1621,11 @@ test_cg(is_boolean, [#k_atom{val=Val}], Fail, I, Vdb, Bef, St) ->
 	     false -> [{jump,{f,Fail}}]
 	 end,
     {Is,Aft,St};
+test_cg(is_map_key, As, Fail, I, Vdb, Bef, St) ->
+    [Key,Map] = cg_reg_args(As, Bef),
+    Aft = clear_dead(Bef, I, Vdb),
+    F = {f,Fail},
+    {[{test,is_map,F,[Map]},{test,has_map_fields,F,Map,{list,[Key]}}],Aft,St};
 test_cg(Test, As, Fail, I, Vdb, Bef, St) ->
     Args = cg_reg_args(As, Bef),
     Aft = clear_dead(Bef, I, Vdb),
diff --git a/lib/compiler/test/map_SUITE.erl b/lib/compiler/test/map_SUITE.erl
index e98c295da6..6badc7a8b8 100644
--- a/lib/compiler/test/map_SUITE.erl
+++ b/lib/compiler/test/map_SUITE.erl
@@ -1203,12 +1203,18 @@ t_guard_bifs(Config) when is_list(Config) ->
     true   = map_guard_empty_2(),
     true   = map_guard_head(#{a=>1}),
     false  = map_guard_head([]),
+    true   = map_get_head(#{a=>1}),
+    false  = map_get_head([]),
+    true   = map_is_key_head(#{a=>1}),
+    false  = map_is_key_head(#{}),
     true   = map_guard_body(#{a=>1}),
     false  = map_guard_body({}),
     true   = map_guard_pattern(#{a=>1, <<"hi">> => "hi" }),
     false  = map_guard_pattern("list"),
     true   = map_guard_tautology(),
     true   = map_guard_ill_map_size(),
+    true   = map_field_check_sequence(#{a=>1}),
+    false  = map_field_check_sequence(#{}),
     ok.
 
 map_guard_empty() when is_map(#{}); false -> true.
@@ -1218,6 +1224,12 @@ map_guard_empty_2() when true; #{} andalso false -> true.
 map_guard_head(M) when is_map(M) -> true;
 map_guard_head(_) -> false.
 
+map_get_head(M) when map_get(a, M) =:= 1 -> true;
+map_get_head(_) -> false.
+
+map_is_key_head(M) when is_map_key(a, M) -> true;
+map_is_key_head(M) -> false.
+
 map_guard_body(M) -> is_map(M).
 
 map_guard_pattern(#{}) -> true;
@@ -1227,6 +1239,12 @@ map_guard_tautology() when #{} =:= #{}; true -> true.
 
 map_guard_ill_map_size() when true; map_size(0) -> true.
 
+map_field_check_sequence(M)
+  when is_map(M) andalso is_map_key(a, M) andalso (map_get(a, M) == 1) ->
+    true;
+map_field_check_sequence(_) ->
+    false.
+
 t_guard_sequence(Config) when is_list(Config) ->
 	{1, "a"} = map_guard_sequence_1(#{seq=>1,val=>id("a")}),
 	{2, "b"} = map_guard_sequence_1(#{seq=>2,val=>id("b")}),
diff --git a/lib/hipe/cerl/erl_bif_types.erl b/lib/hipe/cerl/erl_bif_types.erl
index fe6ab0659c..48ce641ab9 100644
--- a/lib/hipe/cerl/erl_bif_types.erl
+++ b/lib/hipe/cerl/erl_bif_types.erl
@@ -665,6 +665,8 @@ type(erlang, is_map, 1, Xs, Opaques) ->
 	    check_guard(X, fun (Y) -> t_is_map(Y, Opaques) end,
 	    t_map(), Opaques) end,
   strict(erlang, is_map, 1, Xs, Fun, Opaques);
+type(erlang, is_map_key, 2, Xs, Opaques) ->
+  type(maps, is_key, 2, Xs, Opaques);
 type(erlang, is_number, 1, Xs, Opaques) ->
   Fun = fun (X) ->
 	    check_guard(X, fun (Y) -> t_is_number(Y, Opaques) end,
@@ -2374,6 +2376,8 @@ arg_types(erlang, is_list, 1) ->
   [t_any()];
 arg_types(erlang, is_map, 1) ->
   [t_any()];
+arg_types(erlang, is_map_key, 2) ->
+  [t_any(), t_map()];
 arg_types(erlang, is_number, 1) ->
   [t_any()];
 arg_types(erlang, is_pid, 1) ->
@@ -2396,7 +2400,7 @@ arg_types(erlang, map_size, 1) ->
   [t_map()];
 %% Guard bif, needs to be here.
 arg_types(erlang, map_get, 2) ->
-  [t_map(), t_any()];
+  [t_any(), t_map()];
 arg_types(erlang, make_fun, 3) ->
   [t_atom(), t_atom(), t_arity()];
 arg_types(erlang, make_tuple, 2) ->
diff --git a/lib/stdlib/src/erl_internal.erl b/lib/stdlib/src/erl_internal.erl
index 6d3d5baa23..dd509191ef 100644
--- a/lib/stdlib/src/erl_internal.erl
+++ b/lib/stdlib/src/erl_internal.erl
@@ -109,6 +109,7 @@ new_type_test(is_function, 2) -> true;
 new_type_test(is_integer, 1) -> true;
 new_type_test(is_list, 1) -> true;
 new_type_test(is_map, 1) -> true;
+new_type_test(is_map_key, 2) -> true;
 new_type_test(is_number, 1) -> true;
 new_type_test(is_pid, 1) -> true;
 new_type_test(is_port, 1) -> true;
@@ -315,6 +316,7 @@ bif(is_function, 2) -> true;
 bif(is_integer, 1) -> true;
 bif(is_list, 1) -> true;
 bif(is_map, 1) -> true;
+bif(is_map_key, 2) -> true;
 bif(is_number, 1) -> true;
 bif(is_pid, 1) -> true;
 bif(is_port, 1) -> true;
diff --git a/lib/stdlib/src/ms_transform.erl b/lib/stdlib/src/ms_transform.erl
index ec8cfd56c2..428c23524b 100644
--- a/lib/stdlib/src/ms_transform.erl
+++ b/lib/stdlib/src/ms_transform.erl
@@ -929,6 +929,7 @@ bool_test(is_port,1) -> true;
 bool_test(is_reference,1) -> true;
 bool_test(is_tuple,1) -> true;
 bool_test(is_map,1) -> true;
+bool_test(is_map_key, 2) -> true;
 bool_test(is_binary,1) -> true;
 bool_test(is_function,1) -> true;
 bool_test(is_record,2) -> true;
diff --git a/lib/tools/emacs/erlang.el b/lib/tools/emacs/erlang.el
index fd51aca861..e08db0ea79 100644
--- a/lib/tools/emacs/erlang.el
+++ b/lib/tools/emacs/erlang.el
@@ -804,6 +804,7 @@ resulting regexp is surrounded by \\_< and \\_>."
       "is_integer"
       "is_list"
       "is_map"
+      "is_map_key"
       "is_number"
       "is_pid"
       "is_port"