Merge pull request #1810 from tomas-abrahamsson/bjorn/preprocessor-extensions

Add ?OTP_RELEASE, -if and -elif to the preprocessor

OTP-15087

6 files changed, 342 insertions, 36 deletions

diff --git a/lib/stdlib/src/epp.erl b/lib/stdlib/src/epp.erl
index 77cc88eb08..cc34d4bdd3 100644
--- a/lib/stdlib/src/epp.erl
+++ b/lib/stdlib/src/epp.erl
@@ -38,7 +38,7 @@
 -type epp_handle() :: pid().
 -type source_encoding() :: latin1 | utf8.
 
--type ifdef() :: 'ifdef' | 'ifndef' | 'else'.
+-type ifdef() :: 'ifdef' | 'ifndef' | 'if' | 'else'.
 
 -type name() :: atom().
 -type argspec() :: 'none'                       %No arguments
@@ -221,6 +221,8 @@ format_error({illegal_function,Macro}) ->
     io_lib:format("?~s can only be used within a function", [Macro]);
 format_error({illegal_function_usage,Macro}) ->
     io_lib:format("?~s must not begin a form", [Macro]);
+format_error(elif_after_else) ->
+    "'elif' following 'else'";
 format_error({'NYI',What}) ->
     io_lib:format("not yet implemented '~s'", [What]);
 format_error({error,Term}) ->
@@ -571,6 +573,7 @@ init_server(Pid, Name, Options, St0) ->
 predef_macros(File) ->
     Machine = list_to_atom(erlang:system_info(machine)),
     Anno = line1(),
+    OtpVersion = list_to_integer(erlang:system_info(otp_release)),
     Defs = [{'FILE', 	           {none,[{string,Anno,File}]}},
 	    {'FUNCTION_NAME',      undefined},
 	    {'FUNCTION_ARITY',     undefined},
@@ -580,7 +583,8 @@ predef_macros(File) ->
 	    {'BASE_MODULE',	   undefined},
 	    {'BASE_MODULE_STRING', undefined},
 	    {'MACHINE',	           {none,[{atom,Anno,Machine}]}},
-	    {Machine,	           {none,[{atom,Anno,true}]}}
+	    {Machine,	           {none,[{atom,Anno,true}]}},
+	    {'OTP_RELEASE',	   {none,[{integer,Anno,OtpVersion}]}}
 	   ],
     maps:from_list(Defs).
 
@@ -1085,21 +1089,118 @@ scan_else(_Toks, Else, From, St) ->
     epp_reply(From, {error,{loc(Else),epp,{bad,'else'}}}),
     wait_req_scan(St).
 
-%% scan_if(Tokens, EndifToken, From, EppState)
+%% scan_if(Tokens, IfToken, From, EppState)
 %%  Handle the conditional parsing of a file.
-%%  Report a badly formed if test and then treat as false macro.
 
+scan_if([{'(',_}|_]=Toks, If, From, St) ->
+    try eval_if(Toks, St) of
+	true ->
+	    scan_toks(From, St#epp{istk=['if'|St#epp.istk]});
+	_ ->
+	    skip_toks(From, St, ['if'])
+    catch
+	throw:Error0 ->
+	    Error = case Error0 of
+			{_,erl_parse,_} ->
+			    {error,Error0};
+			_ ->
+			    {error,{loc(If),epp,Error0}}
+		    end,
+	    epp_reply(From, Error),
+	    wait_req_skip(St, ['if'])
+    end;
 scan_if(_Toks, If, From, St) ->
-    epp_reply(From, {error,{loc(If),epp,{'NYI','if'}}}),
+    epp_reply(From, {error,{loc(If),epp,{bad,'if'}}}),
     wait_req_skip(St, ['if']).
 
+eval_if(Toks0, St) ->
+    Toks = expand_macros(Toks0, St),
+    Es1 = case erl_parse:parse_exprs(Toks) of
+	      {ok,Es0} -> Es0;
+	      {error,E} -> throw(E)
+	  end,
+    Es = rewrite_expr(Es1, St),
+    assert_guard_expr(Es),
+    Bs = erl_eval:new_bindings(),
+    LocalFun = fun(_Name, _Args) ->
+		       error(badarg)
+	       end,
+    try erl_eval:exprs(Es, Bs, {value,LocalFun}) of
+	{value,Res,_} ->
+	    Res
+    catch
+	_:_ ->
+	    false
+    end.
+
+assert_guard_expr([E0]) ->
+    E = rewrite_expr(E0, none),
+    case erl_lint:is_guard_expr(E) of
+	false ->
+	    throw({bad,'if'});
+	true ->
+	    ok
+    end;
+assert_guard_expr(_) ->
+    throw({bad,'if'}).
+
+%% Dual-purpose rewriting function. When the second argument is
+%% an #epp{} record, calls to defined(Symbol) will be evaluated.
+%% When the second argument is 'none', legal calls to our built-in
+%% functions are eliminated in order to turn the expression into
+%% a legal guard expression.
+
+rewrite_expr({call,_,{atom,_,defined},[N0]}, #epp{macs=Macs}) ->
+    %% Evaluate defined(Symbol).
+    N = case N0 of
+	    {var,_,N1} -> N1;
+	    {atom,_,N1} -> N1;
+	    _ -> throw({bad,'if'})
+	end,
+    {atom,0,maps:is_key(N, Macs)};
+rewrite_expr({call,_,{atom,_,Name},As0}, none) ->
+    As = rewrite_expr(As0, none),
+    Arity = length(As),
+    case erl_internal:bif(Name, Arity) andalso
+	not erl_internal:guard_bif(Name, Arity) of
+	false ->
+	    %% A guard BIF, an -if built-in, or an unknown function.
+	    %% Eliminate the call so that erl_lint will not complain.
+	    %% The call might fail later at evaluation time.
+	    to_conses(As);
+	true ->
+	    %% An auto-imported BIF (not guard BIF). Not allowed.
+	    throw({bad,'if'})
+    end;
+rewrite_expr([H|T], St) ->
+    [rewrite_expr(H, St)|rewrite_expr(T, St)];
+rewrite_expr(Tuple, St) when is_tuple(Tuple) ->
+    list_to_tuple(rewrite_expr(tuple_to_list(Tuple), St));
+rewrite_expr(Other, _) ->
+    Other.
+
+to_conses([H|T]) ->
+    {cons,0,H,to_conses(T)};
+to_conses([]) ->
+    {nil,0}.
+
 %% scan_elif(Tokens, EndifToken, From, EppState)
 %%  Handle the conditional parsing of a file.
 %%  Report a badly formed if test and then treat as false macro.
 
 scan_elif(_Toks, Elif, From, St) ->
-    epp_reply(From, {error,{loc(Elif),epp,{'NYI','elif'}}}),
-    wait_req_scan(St).
+    case St#epp.istk of
+	['else'|Cis] ->
+	    epp_reply(From, {error,{loc(Elif),
+                                    epp,{illegal,"unbalanced",'elif'}}}),
+	    wait_req_skip(St#epp{istk=Cis}, ['else']);
+	[_I|Cis] ->
+	    skip_toks(From, St#epp{istk=Cis}, ['elif']);
+	[] ->
+	    epp_reply(From, {error,{loc(Elif),epp,
+                                    {illegal,"unbalanced",elif}}}),
+	    wait_req_scan(St)
+    end.
 
 %% scan_endif(Tokens, EndifToken, From, EppState)
 %%  If we are in an if body then exit it, else report an error.
@@ -1158,6 +1259,8 @@ skip_toks(From, St, [I|Sis]) ->
 	    skip_toks(From, St#epp{location=Cl}, ['if',I|Sis]);
 	{ok,[{'-',_Lh},{atom,_Le,'else'}=Else|_Toks],Cl}->
 	    skip_else(Else, From, St#epp{location=Cl}, [I|Sis]);
+	{ok,[{'-',_Lh},{atom,_Le,'elif'}=Elif|Toks],Cl}->
+	    skip_elif(Toks, Elif, From, St#epp{location=Cl}, [I|Sis]);
 	{ok,[{'-',_Lh},{atom,_Le,endif}|_Toks],Cl} ->
 	    skip_toks(From, St#epp{location=Cl}, Sis);
 	{ok,_Toks,Cl} ->
@@ -1188,11 +1291,21 @@ skip_toks(From, St, []) ->
 skip_else(Else, From, St, ['else'|Sis]) ->
     epp_reply(From, {error,{loc(Else),epp,{illegal,"repeated",'else'}}}),
     wait_req_skip(St, ['else'|Sis]);
+skip_else(_Else, From, St, ['elif'|Sis]) ->
+    skip_toks(From, St, ['else'|Sis]);
 skip_else(_Else, From, St, [_I]) ->
     scan_toks(From, St#epp{istk=['else'|St#epp.istk]});
 skip_else(_Else, From, St, Sis) ->
     skip_toks(From, St, Sis).
 
+skip_elif(_Toks, Elif, From, St, ['else'|_]=Sis) ->
+    epp_reply(From, {error,{loc(Elif),epp,elif_after_else}}),
+    wait_req_skip(St, Sis);
+skip_elif(Toks, Elif, From, St, [_I]) ->
+    scan_if(Toks, Elif, From, St);
+skip_elif(_Toks, _Elif, From, St, Sis) ->
+    skip_toks(From, St, Sis).
+
 %% macro_pars(Tokens, ArgStack)
 %% macro_expansion(Tokens, Anno)
 %%  Extract the macro parameters and the expansion from a macro definition.
diff --git a/lib/stdlib/test/epp_SUITE.erl b/lib/stdlib/test/epp_SUITE.erl
index 9123bf2f28..a3e294ffea 100644
--- a/lib/stdlib/test/epp_SUITE.erl
+++ b/lib/stdlib/test/epp_SUITE.erl
@@ -28,7 +28,8 @@
          otp_8130/1, overload_mac/1, otp_8388/1, otp_8470/1,
          otp_8562/1, otp_8665/1, otp_8911/1, otp_10302/1, otp_10820/1,
          otp_11728/1, encoding/1, extends/1,  function_macro/1,
-	 test_error/1, test_warning/1, otp_14285/1]).
+	 test_error/1, test_warning/1, otp_14285/1,
+	 test_if/1]).
 
 -export([epp_parse_erl_form/2]).
 
@@ -69,7 +70,7 @@ all() ->
      overload_mac, otp_8388, otp_8470, otp_8562,
      otp_8665, otp_8911, otp_10302, otp_10820, otp_11728,
      encoding, extends, function_macro, test_error, test_warning,
-     otp_14285].
+     otp_14285, test_if].
 
 groups() -> 
     [{upcase_mac, [], [upcase_mac_1, upcase_mac_2]},
@@ -799,7 +800,8 @@ otp_8130(Config) when is_list(Config) ->
     PreDefMacs = macs(Epp),
     ['BASE_MODULE','BASE_MODULE_STRING','BEAM','FILE',
      'FUNCTION_ARITY','FUNCTION_NAME',
-     'LINE','MACHINE','MODULE','MODULE_STRING'] = PreDefMacs,
+     'LINE','MACHINE','MODULE','MODULE_STRING',
+     'OTP_RELEASE'] = PreDefMacs,
     {ok,[{'-',_},{atom,_,file}|_]} = epp:scan_erl_form(Epp),
     {ok,[{'-',_},{atom,_,module}|_]} = epp:scan_erl_form(Epp),
     {ok,[{atom,_,t}|_]} = epp:scan_erl_form(Epp),
@@ -952,27 +954,7 @@ ifdef(Config) ->
 
           {define_c5,
            <<"-\ndefine a.\n">>,
-           {errors,[{{2,1},epp,{bad,define}}],[]}},
-
-          {define_c6,
-           <<"\n-if.\n"
-             "-endif.\n">>,
-           {errors,[{{2,2},epp,{'NYI','if'}}],[]}},
-
-          {define_c7,
-           <<"-ifndef(a).\n"
-             "-elif.\n"
-             "-endif.\n">>,
-           {errors,[{{2,2},epp,{'NYI',elif}}],[]}},
-
-          {define_c7,
-           <<"-ifndef(a).\n"
-             "-if.\n"
-             "-elif.\n"
-             "-endif.\n"
-             "-endif.\n"
-             "t() -> a.\n">>,
-           {errors,[{{2,2},epp,{'NYI','if'}}],[]}}
+           {errors,[{{2,1},epp,{bad,define}}],[]}}
           ],
     [] = compile(Config, Cs),
 
@@ -1117,6 +1099,147 @@ test_warning(Config) ->
     [] = compile(Config, Cs),
     ok.
 
+%% OTP-12847: Test the -if and -elif directives and the built-in
+%% function defined(Symbol).
+test_if(Config) ->
+    Cs = [{if_1c,
+	   <<"-if.\n"
+	     "-endif.\n"
+	     "-if no_parentheses.\n"
+	     "-endif.\n"
+	     "-if(syntax error.\n"
+	     "-endif.\n"
+	     "-if(true).\n"
+	     "-if(a+3).\n"
+	     "syntax error not triggered here.\n"
+	     "-endif.\n">>,
+           {errors,[{1,epp,{bad,'if'}},
+		    {3,epp,{bad,'if'}},
+		    {5,erl_parse,["syntax error before: ","error"]},
+		    {11,epp,{illegal,"unterminated",'if'}}],
+	    []}},
+
+	  {if_2c,			       	%Bad guard expressions.
+	   <<"-if(is_list(integer_to_list(42))).\n" %Not guard BIF.
+	     "-endif.\n"
+	     "-if(begin true end).\n"
+	     "-endif.\n">>,
+	   {errors,[{1,epp,{bad,'if'}},
+		    {3,epp,{bad,'if'}}],
+	    []}},
+
+	  {if_3c,			       	%Invalid use of defined/1.
+	   <<"-if defined(42).\n"
+	     "-endif.\n">>,
+	   {errors,[{1,epp,{bad,'if'}}],[]}},
+
+	  {if_4c,
+	   <<"-elif OTP_RELEASE > 18.\n">>,
+	   {errors,[{1,epp,{illegal,"unbalanced",'elif'}}],[]}},
+
+	  {if_5c,
+	   <<"-ifdef(not_defined_today).\n"
+	     "-else.\n"
+	     "-elif OTP_RELEASE > 18.\n"
+	     "-endif.\n">>,
+	   {errors,[{3,epp,{illegal,"unbalanced",'elif'}}],[]}},
+
+	  {if_6c,
+	   <<"-if(defined(OTP_RELEASE)).\n"
+	     "-else.\n"
+	     "-elif(true).\n"
+	     "-endif.\n">>,
+	   {errors,[{3,epp,elif_after_else}],[]}},
+
+	  {if_7c,
+	   <<"-if(begin true end).\n"		%Not a guard expression.
+	     "-endif.\n">>,
+	   {errors,[{1,epp,{bad,'if'}}],[]}}
+
+	 ],
+    [] = compile(Config, Cs),
+
+    Ts = [{if_1,
+	   <<"-if(?OTP_RELEASE > 18).\n"
+	     "t() -> ok.\n"
+	     "-else.\n"
+	     "a bug.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_2,
+	   <<"-if(false).\n"
+	     "a bug.\n"
+	     "-elif(?OTP_RELEASE > 18).\n"
+	     "t() -> ok.\n"
+	     "-else.\n"
+	     "a bug.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_3,
+	   <<"-if(true).\n"
+	     "t() -> ok.\n"
+	     "-elif(?OTP_RELEASE > 18).\n"
+	     "a bug.\n"
+	     "-else.\n"
+	     "a bug.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_4,
+	   <<"-define(a, 1).\n"
+	     "-if(defined(a) andalso defined(OTP_RELEASE)).\n"
+	     "t() -> ok.\n"
+	     "-else.\n"
+	     "a bug.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_5,
+	   <<"-if(defined(a)).\n"
+	     "a bug.\n"
+	     "-else.\n"
+	     "t() -> ok.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_6,
+	   <<"-if(defined(not_defined_today)).\n"
+	     " -if(true).\n"
+	     "  bug1.\n"
+	     " -elif(true).\n"
+	     "  bug2.\n"
+	     " -elif(true).\n"
+	     "  bug3.\n"
+	     " -else.\n"
+	     "  bug4.\n"
+	     " -endif.\n"
+	     "-else.\n"
+	     "t() -> ok.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_7,
+	   <<"-if(not_builtin()).\n"
+	     "a bug.\n"
+	     "-else.\n"
+	     "t() -> ok.\n"
+	     "-endif.\n">>,
+           ok},
+
+	  {if_8,
+	   <<"-if(42).\n"			%Not boolean.
+	     "a bug.\n"
+	     "-else.\n"
+	     "t() -> ok.\n"
+	     "-endif.\n">>,
+	   ok}
+	 ],
+    [] = run(Config, Ts),
+
+    ok.
+
 %% Advanced test on overloading macros.
 overload_mac(Config) when is_list(Config) ->
     Cs = [
diff --git a/lib/syntax_tools/src/epp_dodger.erl b/lib/syntax_tools/src/epp_dodger.erl
index 0a12e8fd8b..7e741cc649 100644
--- a/lib/syntax_tools/src/epp_dodger.erl
+++ b/lib/syntax_tools/src/epp_dodger.erl
@@ -502,6 +502,10 @@ quickscan_form([{'-', _L}, {atom, La, ifdef} | _Ts]) ->
     kill_form(La);
 quickscan_form([{'-', _L}, {atom, La, ifndef} | _Ts]) ->
     kill_form(La);
+quickscan_form([{'-', _L}, {'if', La} | _Ts]) ->
+    kill_form(La);
+quickscan_form([{'-', _L}, {atom, La, elif} | _Ts]) ->
+    kill_form(La);
 quickscan_form([{'-', _L}, {atom, La, else} | _Ts]) ->
     kill_form(La);
 quickscan_form([{'-', _L}, {atom, La, endif} | _Ts]) ->
@@ -615,8 +619,13 @@ filter_form(T) ->
 %% ---------------------------------------------------------------------
 %% Normal parsing - try to preserve all information
 
-normal_parser(Ts, Opt) ->
-    rewrite_form(parse_tokens(scan_form(Ts, Opt))).
+normal_parser(Ts0, Opt) ->
+    case scan_form(Ts0, Opt) of
+	Ts when is_list(Ts) ->
+	    rewrite_form(parse_tokens(Ts));
+	Node ->
+	    Node
+    end.
 
 scan_form([{'-', _L}, {atom, La, define} | Ts], Opt) ->
     [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
@@ -636,12 +645,26 @@ scan_form([{'-', _L}, {atom, La, ifdef} | Ts], Opt) ->
 scan_form([{'-', _L}, {atom, La, ifndef} | Ts], Opt) ->
     [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
      {atom, La, ifndef} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {'if', La} | Ts], Opt) ->
+    [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
+     {atom, La, 'if'} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {atom, La, elif} | Ts], Opt) ->
+    [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
+     {atom, La, 'elif'} | scan_macros(Ts, Opt)];
 scan_form([{'-', _L}, {atom, La, else} | Ts], Opt) ->
     [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
      {atom, La, else} | scan_macros(Ts, Opt)];
 scan_form([{'-', _L}, {atom, La, endif} | Ts], Opt) ->
     [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
      {atom, La, endif} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {atom, La, error} | Ts], _Opt) ->
+    Desc = build_info_string("-error", Ts),
+    ErrorInfo = {La, ?MODULE, {error, Desc}},
+    erl_syntax:error_marker(ErrorInfo);
+scan_form([{'-', _L}, {atom, La, warning} | Ts], _Opt) ->
+    Desc = build_info_string("-warning", Ts),
+    ErrorInfo = {La, ?MODULE, {warning, Desc}},
+    erl_syntax:error_marker(ErrorInfo);
 scan_form([{'-', L}, {'?', L1}, {Type, _, _}=N | [{'(', _} | _]=Ts], Opt)
   when Type =:= atom; Type =:= var ->
     %% minus, macro and open parenthesis at start of form - assume that
@@ -657,6 +680,11 @@ scan_form([{'?', L}, {Type, _, _}=N | [{'(', _} | _]=Ts], Opt)
 scan_form(Ts, Opt) ->
     scan_macros(Ts, Opt).
 
+build_info_string(Prefix, Ts0) ->
+    Ts = lists:droplast(Ts0),
+    String = lists:droplast(tokens_to_string(Ts)),
+    Prefix ++ " " ++ String ++ ".".
+
 scan_macros(Ts, Opt) ->
     scan_macros(Ts, [], Opt).
 
@@ -865,6 +893,10 @@ tokens_to_string([]) ->
 
 format_error(macro_args) ->
     errormsg("macro call missing end parenthesis");
+format_error({error, Error}) ->
+    Error;
+format_error({warning, Error}) ->
+    Error;
 format_error({unknown, Reason}) ->
     errormsg(io_lib:format("unknown error: ~tP", [Reason, 15])).
 
diff --git a/lib/syntax_tools/src/erl_prettypr.erl b/lib/syntax_tools/src/erl_prettypr.erl
index 60a15c8e3f..6906ef1553 100644
--- a/lib/syntax_tools/src/erl_prettypr.erl
+++ b/lib/syntax_tools/src/erl_prettypr.erl
@@ -675,7 +675,12 @@ lay_2(Node, Ctxt) ->
 	    %% attribute name, without following parentheses.
 	    Ctxt1 = reset_prec(Ctxt),
             Args = erl_syntax:attribute_arguments(Node),
-            N = erl_syntax:attribute_name(Node),
+            N = case erl_syntax:attribute_name(Node) of
+                    {atom, _, 'if'} ->
+                        erl_syntax:variable('if');
+                    N0 ->
+                        N0
+                end,
             D = case attribute_type(Node) of
                     spec ->
                         [SpecTuple] = Args,
diff --git a/lib/syntax_tools/src/erl_syntax_lib.erl b/lib/syntax_tools/src/erl_syntax_lib.erl
index c7f477c4d2..ced0dba3e2 100644
--- a/lib/syntax_tools/src/erl_syntax_lib.erl
+++ b/lib/syntax_tools/src/erl_syntax_lib.erl
@@ -1317,6 +1317,8 @@ analyze_attribute(Node) ->
                 include_lib -> preprocessor;
                 ifdef -> preprocessor;
                 ifndef -> preprocessor;
+                'if' -> preprocessor;
+                elif -> preprocessor;
                 else -> preprocessor;
                 endif -> preprocessor;
                 A ->
diff --git a/system/doc/reference_manual/macros.xml b/system/doc/reference_manual/macros.xml
index a341307ab7..760599308c 100644
--- a/system/doc/reference_manual/macros.xml
+++ b/system/doc/reference_manual/macros.xml
@@ -150,6 +150,11 @@ bar(X) ->
       <item>The name of the current function.</item>
       <tag><c>?FUNCTION_ARITY</c></tag>
       <item>The arity (number of arguments) for the current function.</item>
+      <tag><c>?OTP_RELEASE</c></tag>
+      <item>The OTP release that the currently executing ERTS
+        application is part of, as an integer. For details, see
+        <seealso marker="erts:erlang#system_info/1"><c>erlang:system_info(otp_release)</c></seealso>.
+        This macro was introduced in OTP release 21.</item>
     </taglist>
   </section>
 
@@ -202,8 +207,16 @@ f() ->
        directive. If that condition is false, the lines following
       <c>else</c> are evaluated instead.</item>
       <tag><c>-endif.</c></tag>
-      <item>Specifies the end of an <c>ifdef</c> or <c>ifndef</c>
-       directive.</item>
+      <item>Specifies the end of an <c>ifdef</c>, an <c>ifndef</c>
+       directive, or the end of an <c>if</c> or <c>elif</c> directive.</item>
+      <tag><c>-if(Condition).</c></tag>
+      <item>Evaluates the following lines only if <c>Condition</c>
+       evaluates to true.</item>
+      <tag><c>-elif(Condition).</c></tag>
+      <item>Only allowed after an <c>if</c> or another <c>elif</c> directive.
+       If the preceding <c>if</c> or <c>elif</c> directives do not
+       evaluate to true, and the <c>Condition</c> evaluates to true,
+       the lines following the <c>elif</c> are evaluated instead.</item>
     </taglist>
     <note>
       <p>The macro directives cannot be used inside functions.</p>
@@ -231,6 +244,24 @@ or
 {ok,m}</pre>
     <p><c>?LOG(Arg)</c> is then expanded to a call to <c>io:format/2</c>
       and provide the user with some simple trace output.</p>
+
+    <p><em>Example:</em></p>
+    <code type="none">
+-module(m)
+...
+-ifdef(OTP_RELEASE).
+  %% OTP 21 or higher
+  -if(?OTP_RELEASE >= 22).
+    %% Code that will work in OTP 22 or higher
+  -elif(?OTP_RELEASE >= 21).
+    %% Code that will work in OTP 21 or higher
+  -endif.
+-else.
+  %% OTP 20 or lower.
+-endif.
+...</code>
+    <p>The code uses the <c>OTP_RELEASE</c> macro to conditionally
+    select code depending on release.</p>
   </section>
 
   <section>