@id required in dictionary files only when @messages is specified.

@id defines an application identifier and this is used only when sending
or receiving messages. A dictionary can define only AVP's however,
to be included by other dictionaries using @inherits, in which case it
makes no sense to require @id.

Note that message definitions are not inherited with @inherits, only
AVP's

lib/diameter/doc/src/diameter_dict.xml

@@ -105,7 +105,7 @@ quantity is insignificant.</p>
 <p>
 The tags, their arguments and the contents of each corresponding
 section are as follows.
-Each section can occur only once unless otherwise specified.
+Each section can occur at most once unless otherwise specified.
 The order in which sections are specified is unimportant.</p>
 
 <taglist>
@@ -115,6 +115,7 @@ The order in which sections are specified is unimportant.</p>
 <p>
 Defines the integer Number as the Diameter Application Id of the
 application in question.
+Required if the dictionary defines <c>@messages</c>.
 The section has empty content.</p>
 
 <p>

lib/diameter/src/compiler/diameter_codegen.erl

@@ -250,9 +250,14 @@ f_name(Name) ->
 %%% ------------------------------------------------------------------------
 
 f_id(Spec) ->
-    Id = orddict:fetch(id, Spec),
     {?function, id, 0,
-     [{?clause, [], [], [?INTEGER(Id)]}]}.
+     [c_id(orddict:find(id, Spec))]}.
+
+c_id({ok, Id}) ->
+    {?clause, [], [], [?INTEGER(Id)]};
+
+c_id(error) ->
+    ?UNEXPECTED(0).
 
 %%% ------------------------------------------------------------------------
 %%% # vendor_id/0
@@ -537,10 +542,14 @@ f_msg_header(Spec) ->
     {?function, msg_header, 1, msg_header(Spec) ++ [?UNEXPECTED(1)]}.
 
 msg_header(Spec) ->
+    msg_header(get_value(messages, Spec), Spec).
+
+msg_header([], _) ->
+    [];
+msg_header(Msgs, Spec) ->
     ApplId = orddict:fetch(id, Spec),
 
-    lists:map(fun({M,C,F,_,_}) -> c_msg_header(M, C, F, ApplId) end,
-              get_value(messages, Spec)).
+    lists:map(fun({M,C,F,_,_}) -> c_msg_header(M, C, F, ApplId) end, Msgs).
 
 %% Note that any application id in the message header spec is ignored.