Teach erlang:is_builtin/3 that erlang:apply/3 is built-in

erlang:is_builtin(erlang, apply, 3) returns 'false'. That seems to be
an oversight in the implementation of erlang:is_builtin/3 rather than
a conscious design decision. Part of apply/3 is implemented in C (as a
special instruction), and part of it in Erlang (only used if apply/3
is used recursively). That makes apply/3 special compared to all other
BIFs.

From the viewpoint of the user, apply/3 is a built-in function,
since it cannot possibly be implemented in pure Erlang.

Noticed-by: Stavros Aronis

2 files changed, 28 insertions, 2 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index a42ce1c9f1..faf496a030 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -6752,6 +6752,15 @@ erts_is_builtin(Eterm Mod, Eterm Name, int arity)
     Export e;
     Export* ep;
 
+    if (Mod == am_erlang && Name == am_apply && arity == 3) {
+	/*
+	 * Special case. apply/3 is built-in (implemented in C),
+	 * but implemented in a different way than all other
+	 * BIFs.
+	 */
+	return 1;
+    }
+
     e.code[0] = Mod;
     e.code[1] = Name;
     e.code[2] = arity;
diff --git a/erts/emulator/test/bif_SUITE.erl b/erts/emulator/test/bif_SUITE.erl
index d6a771e7b9..ebc4db53c4 100644
--- a/erts/emulator/test/bif_SUITE.erl
+++ b/erts/emulator/test/bif_SUITE.erl
@@ -32,7 +32,8 @@
 	 specs/1,improper_bif_stubs/1,auto_imports/1,
 	 t_list_to_existing_atom/1,os_env/1,otp_7526/1,
 	 binary_to_atom/1,binary_to_existing_atom/1,
-	 atom_to_binary/1,min_max/1, erlang_halt/1]).
+	 atom_to_binary/1,min_max/1, erlang_halt/1,
+	 is_builtin/1]).
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
@@ -42,7 +43,7 @@ all() ->
      t_list_to_existing_atom, os_env, otp_7526,
      display,
      atom_to_binary, binary_to_atom, binary_to_existing_atom,
-     min_max, erlang_halt].
+     min_max, erlang_halt, is_builtin].
 
 groups() -> 
     [].
@@ -716,6 +717,22 @@ wait_until_stable_size(File,PrevSz) ->
             wait_until_stable_size(File,NewSz)
     end.
 
+is_builtin(_Config) ->
+    Exp0 = [{M,F,A} || {M,_} <- code:all_loaded(),
+		       {F,A} <- M:module_info(exports)],
+    Exp = ordsets:from_list(Exp0),
+
+    %% erlang:apply/3 is considered to be built-in, but is not
+    %% implemented as other BIFs.
+
+    Builtins0 = [{erlang,apply,3}|erlang:system_info(snifs)],
+    Builtins = ordsets:from_list(Builtins0),
+    NotBuiltin = ordsets:subtract(Exp, Builtins),
+    _ = [true = erlang:is_builtin(M, F, A) || {M,F,A} <- Builtins],
+    _ = [false = erlang:is_builtin(M, F, A) || {M,F,A} <- NotBuiltin],
+
+    ok.
+
 
 %% Helpers