Fix regression: completion for incomplete export enrty

The unexpected atom (an incomplete function name) in the export attribute caused
the below crash in the parser. This way the completion provider failed to
determine that the location is in an export context, and used function snippet
with arguments instead of '/arity'.

For example when hitting enter on a completion erlang_ls used to insert
`foo(Arg)` instead of `foo/1`.

```
1> els_parser:parse("-export([foo]).").
2021-06-06T16:40:30.522244+02:00 warning: Please report error parsing form error:{try_clause,foo}:[{els_parser,'-skip_function_entries/1-fun-0-',1,[{file,"/Users/ext.pgomori/git/erlang_ls/apps/els_lsp/src/els_parser.erl"},{line,839}]},{lists,'-filter/2-lc$^0/1-0-',2,[{file,"lists.erl"},{line,1286}]},{els_parser,attribute_subtrees,2,[{file,"/Users/ext.pgomori/git/erlang_ls/apps/els_lsp/src/els_parser.erl"},{line,792}]},{els_parser,fold,3,[{file,"/Users/ext.pgomori/git/erlang_ls/apps/els_lsp/src/els_parser.erl"},{line,687}]},{els_parser,'-parse_forms/1-lc$^0/1-0-',1,[{file,"/Users/ext.pgomori/git/erlang_ls/apps/els_lsp/src/els_parser.erl"},{line,59}]},{els_parser,parse,1,[{file,"/Users/ext.pgomori/git/erlang_ls/apps/els_lsp/src/els_parser.erl"},{line,30}]},{erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,684}]},{shell,exprs,7,[{file,"shell.erl"},{line,686}]}], {attribute,#{end_location => {1,16},location => {1,1}},{atom,#{end_location => {1,8},location => {1,2},text => "export"},export},[{list,#{end_location => {1,14},location => {1,9}},[{atom,#{end_location => {1,13},location => {1,10},text => "foo"},foo}]}]}
```

apps/els_lsp/priv/code_navigation/src/completion_incomplete.erl

@@ -0,0 +1,11 @@
+-module(completion_incomplete).
+
+-export([ function_exported/0
+        , function_
+        ]).
+
+function_exported() ->
+  ok.
+
+function_unexported() ->
+  ok.

apps/els_lsp/src/els_parser.erl

@@ -353,10 +353,10 @@ find_export_pois(Tree, AttrName, Arg) ->
                             -> [poi()].
 find_export_entry_pois(EntryPoiKind, Exports) ->
   lists:flatten(
-    [ try erl_syntax_lib:analyze_function_name(FATree) of
+    [ case get_name_arity(FATree) of
         {F, A} ->
-          poi(erl_syntax:get_pos(FATree), EntryPoiKind, {F, A})
-      catch throw:syntax_error ->
+          poi(erl_syntax:get_pos(FATree), EntryPoiKind, {F, A});
+        false ->
           []
       end
       || FATree <- Exports
@@ -365,10 +365,10 @@ find_export_entry_pois(EntryPoiKind, Exports) ->
 -spec find_import_entry_pois(atom(), [tree()]) -> [poi()].
 find_import_entry_pois(M, Imports) ->
   lists:flatten(
-    [ try erl_syntax_lib:analyze_function_name(FATree) of
+    [ case get_name_arity(FATree) of
         {F, A} ->
-          poi(erl_syntax:get_pos(FATree), import_entry, {M, F, A})
-      catch throw:syntax_error ->
+          poi(erl_syntax:get_pos(FATree), import_entry, {M, F, A});
+        false ->
           []
       end
       || FATree <- Imports
@@ -668,6 +668,28 @@ is_atom_node(Tree) ->
       false
   end.
 
+-spec get_name_arity(tree()) -> {atom(), integer()} | false.
+get_name_arity(Tree) ->
+  case erl_syntax:type(Tree) of
+    arity_qualifier ->
+      A = erl_syntax:arity_qualifier_argument(Tree),
+      case erl_syntax:type(A) of
+        integer ->
+          F = erl_syntax:arity_qualifier_body(Tree),
+          case is_atom_node(F) of
+            {true, Name} ->
+              Arity = erl_syntax:integer_value(A),
+              {Name, Arity};
+            false ->
+              false
+          end;
+        _ ->
+          false
+      end;
+    _ ->
+      false
+  end.
+
 -spec poi(pos() | {pos(), pos()} | erl_anno:anno(), poi_kind(), any()) -> poi().
 poi(Pos, Kind, Id) ->
   poi(Pos, Kind, Id, undefined).
@@ -836,10 +858,9 @@ skip_function_entries(FunList) ->
     list ->
       lists:filter(
         fun(FATree) ->
-            try erl_syntax_lib:analyze_function_name(FATree) of
-              {_, _} -> false
-            catch
-              throw:syntax_error -> true
+            case get_name_arity(FATree) of
+              {_, _} -> false;
+              false -> true
             end
         end, erl_syntax:list_elements(FunList));
     _ ->

apps/els_lsp/test/els_completion_SUITE.erl

@@ -14,6 +14,7 @@
         , attribute_include/1
         , attribute_include_lib/1
         , attribute_export/1
+        , attribute_export_incomplete/1
         , attribute_export_type/1
         , default_completions/1
         , empty_completions/1
@@ -273,6 +274,36 @@ attribute_export(Config) ->
   [?assertNot(lists:member(E, Completions)) || E <- NotExpected],
   ok.
 
+-spec attribute_export_incomplete(config()) -> ok.
+attribute_export_incomplete(Config) ->
+  Uri = ?config(completion_incomplete_uri, Config),
+  TriggerKindInvoked = ?COMPLETION_TRIGGER_KIND_INVOKED,
+  Expected = [#{ insertTextFormat => ?INSERT_TEXT_FORMAT_PLAIN_TEXT
+               , kind => ?COMPLETION_ITEM_KIND_FUNCTION
+               , label => <<"function_unexported/0">>
+               , data =>
+                   #{ arity => 0
+                    , function => <<"function_unexported">>
+                    , module => <<"completion_incomplete">>
+                    }
+               }
+             ],
+  NotExpected = [#{ insertTextFormat => ?INSERT_TEXT_FORMAT_PLAIN_TEXT
+                  , kind => ?COMPLETION_ITEM_KIND_FUNCTION
+                  , label => <<"function_exported/0">>
+                  , data =>
+                      #{ arity => 0
+                       , function => <<"function_exported">>
+                       , module => <<"completion_incomplete">>
+                       }
+                  }
+                ],
+  #{result := Completions} =
+    els_client:completion(Uri, 4, 18, TriggerKindInvoked, <<"">>),
+  [?assert(lists:member(E, Completions)) || E <- Expected],
+  [?assertNot(lists:member(E, Completions)) || E <- NotExpected],
+  ok.
+
 -spec attribute_export_type(config()) -> ok.
 attribute_export_type(Config) ->
   Uri = ?config(completion_attributes_uri, Config),

apps/els_lsp/test/els_test_utils.erl

@@ -134,6 +134,7 @@ sources() ->
   , completion_resolve_2
   , completion_snippets
   , completion_attributes
+  , completion_incomplete
   , diagnostics
   , diagnostics_bound_var_in_pattern
   , diagnostics_autoimport