Fix bogus bound variable warnings in fun expression heads

This is a workaround for erl_syntax_lib not considering shadowing in fun
expressions.

There is a further issue with named funs where variables from the name and head
are not added to the env of the fun body. This is not fixed by the current
change.

Fixes #982.

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

@@ -1,6 +1,6 @@
 -module(diagnostics_bound_var_in_pattern).
 
--export([f/1, g/1, h/2]).
+-export([f/1, g/1, h/2, fun_expr/1, named_fun_expr/0]).
 
 f(Var1) ->
   Var1 = 1.
@@ -18,3 +18,14 @@ h(Var3, Var4) ->
   catch Var4 ->
       error
   end.
+
+fun_expr(New) ->
+  fun(New, Var5) ->
+      Var5 = New
+  end.
+
+named_fun_expr() ->
+  fun F(New, Var6) ->
+      New = Var6,
+      F = Var6
+  end.

apps/els_lsp/src/els_bound_var_in_pattern_diagnostics.erl

@@ -61,7 +61,7 @@ find_vars_in_form(Form) ->
     function ->
       AnnotatedForm = erl_syntax_lib:annotate_bindings(Form, []),
       %% There are no bound variables in function heads or guards
-      %% so lets decend straight into the bodies
+      %% so lets descend straight into the bodies
       Clauses = erl_syntax:function_clauses(AnnotatedForm),
       ClauseBodies = lists:map(fun erl_syntax:clause_body/1, Clauses),
       fold_subtrees(ClauseBodies, []);
@@ -76,6 +76,19 @@ fold_subtrees(Subtrees, Acc) ->
 -spec find_vars_in_tree(tree(), [poi()]) -> [poi()].
 find_vars_in_tree(Tree, Acc) ->
   case erl_syntax:type(Tree) of
+    Type when Type =:= fun_expr;
+              Type =:= named_fun_expr ->
+      %% There is no bound variables in fun expression heads,
+      %% because they shadow whatever is in the input env
+      %% so lets descend straight into the bodies
+      %% (This is a workaround for erl_syntax_lib not considering
+      %%  shadowing in fun expressions)
+      Clauses = case Type of
+                  fun_expr -> erl_syntax:fun_expr_clauses(Tree);
+                  named_fun_expr -> erl_syntax:named_fun_expr_clauses(Tree)
+                end,
+      ClauseBodies = lists:map(fun erl_syntax:clause_body/1, Clauses),
+      fold_subtrees(ClauseBodies, Acc);
     match_expr ->
       Pattern = erl_syntax:match_expr_pattern(Tree),
       NewAcc = fold_pattern(Pattern, Acc),

apps/els_lsp/test/els_diagnostics_SUITE.erl

@@ -176,10 +176,30 @@ bound_var_in_pattern(Config) ->
           #{'end' => #{character => 12, line => 17},
             start => #{character => 8, line => 17}},
         severity => 4,
+        source => <<"BoundVarInPattern">>},
+      #{message => <<"Bound variable in pattern: Var5">>,
+        range =>
+          #{'end' => #{character => 10, line => 23},
+            start => #{character => 6, line => 23}},
+        severity => 4,
         source => <<"BoundVarInPattern">>}
+      %% erl_syntax_lib:annotate_bindings does not handle named funs correctly
+      %% #{message => <<"Bound variable in pattern: New">>,
+      %%   range =>
+      %%     #{'end' => #{character => 9, line => 28},
+      %%       start => #{character => 6, line => 28}},
+      %%   severity => 4,
+      %%   source => <<"BoundVarInPattern">>},
+      %% #{message => <<"Bound variable in pattern: F">>,
+      %%   range =>
+      %%     #{'end' => #{character => 7, line => 29},
+      %%       start => #{character => 6, line => 29}},
+      %%   severity => 4,
+      %%   source => <<"BoundVarInPattern">>}
     ],
   F = fun(#{message := M1}, #{message := M2}) -> M1 =< M2 end,
-  ?assertEqual(Expected, lists:sort(F, Diagnostics)),
+  Hints = [D || #{severity := ?DIAGNOSTIC_HINT} = D <- Diagnostics],
+  ?assertEqual(Expected, lists:sort(F, Hints)),
   ok.
 
 -spec compiler(config()) -> ok.