dialyzer: Do not emit warnings for unreachable funs

Warnings are not generated for funs residing in dead code.

In particular, warnings like "The created fun has no local return" are
no longer generated for funs declared in clauses or functions that
cannot be run.

3 files changed, 60 insertions, 10 deletions

diff --git a/lib/dialyzer/src/dialyzer_dataflow.erl b/lib/dialyzer/src/dialyzer_dataflow.erl
index c5f93a3392..45b4abb253 100644
--- a/lib/dialyzer/src/dialyzer_dataflow.erl
+++ b/lib/dialyzer/src/dialyzer_dataflow.erl
@@ -102,6 +102,8 @@
                                       | 'undefined', % race
                 fun_homes            :: dict:dict(label(), mfa())
                                       | 'undefined', % race
+                reachable_funs       :: sets:set(label())
+                                      | 'undefined', % race
 		plt		     :: dialyzer_plt:plt()
                                       | 'undefined', % race
 		opaques              :: [type()]
@@ -269,9 +271,11 @@ traverse(Tree, Map, State) ->
       case state__warning_mode(State) of
         true -> {State, Map, Type};
         false ->
-          State2 = state__add_work(get_label(Tree), State),
+          FunLbl = get_label(Tree),
+          State2 = state__add_work(FunLbl, State),
           State3 = state__update_fun_env(Tree, Map, State2),
-          {State3, Map, Type}
+          State4 = state__add_reachable(FunLbl, State3),
+          {State4, Map, Type}
       end;
     'let' ->
       handle_let(Tree, Map, State);
@@ -3039,25 +3043,35 @@ state__new(Callgraph, Codeserver, Tree, Plt, Module, Records) ->
   {TreeMap, FunHomes} = build_tree_map(Tree, Callgraph),
   Funs = dict:fetch_keys(TreeMap),
   FunTab = init_fun_tab(Funs, dict:new(), TreeMap, Callgraph, Plt),
-  ExportedFuns =
-    [Fun || Fun <- Funs--[top], dialyzer_callgraph:is_escaping(Fun, Callgraph)],
-  Work = init_work(ExportedFuns),
+  ExportedFunctions =
+    [Fun ||
+      Fun <- Funs--[top],
+      dialyzer_callgraph:is_escaping(Fun, Callgraph),
+      dialyzer_callgraph:lookup_name(Fun, Callgraph) =/= error
+    ],
+  Work = init_work(ExportedFunctions),
   Env = lists:foldl(fun(Fun, Env) -> dict:store(Fun, map__new(), Env) end,
 		    dict:new(), Funs),
   #state{callgraph = Callgraph, codeserver = Codeserver,
          envs = Env, fun_tab = FunTab, fun_homes = FunHomes, opaques = Opaques,
 	 plt = Plt, races = dialyzer_races:new(), records = Records,
 	 warning_mode = false, warnings = [], work = Work, tree_map = TreeMap,
-	 module = Module}.
+	 module = Module, reachable_funs = sets:new()}.
 
 state__warning_mode(#state{warning_mode = WM}) ->
   WM.
 
 state__set_warning_mode(#state{tree_map = TreeMap, fun_tab = FunTab,
-                               races = Races} = State) ->
+                               races = Races, callgraph = Callgraph,
+                               reachable_funs = ReachableFuns} = State) ->
   ?debug("==========\nStarting warning pass\n==========\n", []),
   Funs = dict:fetch_keys(TreeMap),
-  State#state{work = init_work([top|Funs--[top]]),
+  Work =
+    [Fun ||
+      Fun <- Funs--[top],
+      dialyzer_callgraph:lookup_name(Fun, Callgraph) =/= error orelse
+        sets:is_element(Fun, ReachableFuns)],
+  State#state{work = init_work(Work),
 	      fun_tab = FunTab, warning_mode = true,
               races = dialyzer_races:put_race_analysis(true, Races)}.
 
@@ -3149,7 +3163,8 @@ state__get_race_warnings(#state{races = Races} = State) ->
   State1#state{races = Races1}.
 
 state__get_warnings(#state{tree_map = TreeMap, fun_tab = FunTab,
-			   callgraph = Callgraph, plt = Plt} = State) ->
+			   callgraph = Callgraph, plt = Plt,
+                           reachable_funs = ReachableFuns} = State) ->
   FoldFun =
     fun({top, _}, AccState) -> AccState;
        ({FunLbl, Fun}, AccState) ->
@@ -3184,7 +3199,12 @@ state__get_warnings(#state{tree_map = TreeMap, fun_tab = FunTab,
 		      GenRet = dialyzer_contracts:get_contract_return(C),
 		      not t_is_unit(GenRet)
 		  end,
-		case Warn of
+                %% Do not output warnings for unreachable funs.
+		case
+                  Warn andalso
+                  (dialyzer_callgraph:lookup_name(FunLbl, Callgraph) =/= error
+                   orelse sets:is_element(FunLbl, ReachableFuns))
+                of
 		  true ->
 		    case classify_returns(Fun) of
 		      no_match ->
@@ -3255,6 +3275,10 @@ state__get_args_and_status(Tree, #state{fun_tab = FunTab}) ->
     {ok, {ArgTypes, _}} -> {ArgTypes, true}
   end.
 
+state__add_reachable(FunLbl, #state{reachable_funs = ReachableFuns}=State) ->
+  NewReachableFuns = sets:add_element(FunLbl, ReachableFuns),
+  State#state{reachable_funs = NewReachableFuns}.
+
 build_tree_map(Tree, Callgraph) ->
   Fun =
     fun(T, {Dict, Homes, FunLbls} = Acc) ->
diff --git a/lib/dialyzer/test/small_SUITE_data/results/unused_funs b/lib/dialyzer/test/small_SUITE_data/results/unused_funs
new file mode 100644
index 0000000000..c468457ead
--- /dev/null
+++ b/lib/dialyzer/test/small_SUITE_data/results/unused_funs
@@ -0,0 +1,5 @@
+
+unused_funs.erl:10: The pattern 'error' can never match the type 'other_error'
+unused_funs.erl:15: Function not_used/0 will never be called
+unused_funs.erl:19: Function foo/1 will never be called
+unused_funs.erl:7: Function test/0 has no local return
diff --git a/lib/dialyzer/test/small_SUITE_data/src/unused_funs.erl b/lib/dialyzer/test/small_SUITE_data/src/unused_funs.erl
new file mode 100644
index 0000000000..c24cf3ea81
--- /dev/null
+++ b/lib/dialyzer/test/small_SUITE_data/src/unused_funs.erl
@@ -0,0 +1,21 @@
+%% See also ERL-593.
+
+-module(unused_funs).
+
+-export([test/0]).
+
+test() -> % "has no local return"
+    Var = outer_scope,
+    case other_error of
+        error -> % "can never match"
+            %% No warnings "no local return" and "_ = 1 can never match 0" (!)
+            foo(fun() -> {Var, 1 = 0} end)
+    end.
+
+not_used() -> % "will never be called"
+    %% No warnings "no local return" and "1 can never match 0".
+    foo(fun() -> 1 = 0 end).
+
+foo(Fun) -> % "will never be called"
+    1 = 0, % No pattern match warning (foo/1 is not traversed at all).
+    Fun().