stdlib: Let the linter check unused types properly

See also https://github.com/erlang/otp/issues/4784.

A type like `t() :: [t()]' can now be reported as unused.

Types occurring in specs of unused functions or in unused records are
considered used. This is consistent with how the warning for unused
records is implemented.

1 files changed, 48 insertions, 19 deletions

diff --git a/lib/stdlib/src/erl_lint.erl b/lib/stdlib/src/erl_lint.erl
index 7803273c5e..599f8443ff 100644
--- a/lib/stdlib/src/erl_lint.erl
+++ b/lib/stdlib/src/erl_lint.erl
@@ -93,6 +93,16 @@ value_option(Flag, Default, On, OnVal, Off, OffVal, Opts) ->
 
 -record(typeinfo, {attr, anno}).
 
+-type type_id() :: {'export', []}
+                 | {'record', atom()}
+                 | {'spec', mfa()}
+                 | {'type', ta()}.
+
+-record(used_type, {anno :: erl_anno:anno(),
+                    at = {export, []} :: type_id()}).
+
+-type used_type() :: #used_type{}.
+
 %% Usage of records, functions, and imports. The variable table, which
 %% is passed on as an argument, holds the usage of variables.
 -record(usage, {
@@ -101,7 +111,7 @@ value_option(Flag, Default, On, OnVal, Off, OffVal, Opts) ->
           used_records = gb_sets:new()          %Used record definitions
               :: gb_sets:set(atom()),
           used_types = maps:new()               %Used type definitions
-              :: #{ta() := anno()}
+              :: #{ta() := [used_type()]}
          }).
 
 
@@ -130,6 +140,7 @@ value_option(Flag, Default, On, OnVal, Off, OffVal, Opts) ->
                not_removed=gb_sets:empty()      %Not considered removed
                    :: gb_sets:set(module_or_mfa()),
                func=[],                         %Current function
+               type_id=[],                      %Current type id
                warn_format=0,                   %Warn format calls
 	       enabled_warnings=[],		%All enabled warnings (ordset).
                nowarn_bif_clash=[],             %All no warn bif clashes (ordset).
@@ -1288,10 +1299,10 @@ check_undefined_types(#lint{usage=Usage,types=Def}=St0) ->
 		TA <- UTAs,
 		not is_map_key(TA, Def),
 		not is_default_type(TA)],
-    foldl(fun ({TA,AnnoList}, St) ->
-                  foldl( fun(Anno, St1) ->
+    foldl(fun ({TA,UsedTypeList}, St) ->
+                  foldl( fun(#used_type{anno = Anno}, St1) ->
                                  add_error(Anno, {undefined_type,TA}, St1)
-                         end, St, AnnoList)
+                         end, St, UsedTypeList)
 	  end, St0, Undef).
 
 %% check_bif_clashes(Forms, State0) -> State
@@ -1432,7 +1443,8 @@ export_type(Anno, ETs, #lint{exp_types = ETs0} = St0) ->
 				   Warn = {duplicated_export_type,TA},
 				   add_warning(Anno, Warn, St2);
 			       false ->
-                                   used_type(TA, Anno, St2)
+                                   St3 = St2#lint{type_id = {export, []}},
+                                   used_type(TA, Anno, St3)
 			   end,
 		      {gb_sets:add_element(TA, E), St}
 	      end,
@@ -2674,7 +2686,8 @@ record_def(Anno, Name, Fs0, St0) ->
             St2 = St1#lint{records=maps:put(Name, {Anno,Fs1},
                                             St1#lint.records)},
             Types = [T || {typed_record_field, _, T} <- Fs0],
-            check_type({type, nowarn(), product, Types}, St2)
+            St3 = St2#lint{type_id = {record, Name}},
+            check_type({type, nowarn(), product, Types}, St3)
     end.
 
 %% def_fields([RecDef], RecordName, State) -> {[DefField],State}.
@@ -2894,7 +2907,8 @@ type_def(Attr, Anno, TypeName, ProtoType, Args, St0) ->
         fun(St) ->
                 NewDefs = maps:put(TypePair, Info, TypeDefs),
                 CheckType = {type, nowarn(), product, [ProtoType|Args]},
-                check_type(CheckType, St#lint{types=NewDefs})
+                St1 = St#lint{types=NewDefs, type_id={type, TypePair}},
+                check_type(CheckType, St1)
         end,
     case is_default_type(TypePair) of
         true ->
@@ -3089,7 +3103,9 @@ check_record_types([], _Name, _DefFields, SeenVars, St, _SeenFields) ->
 
 used_type(TypePair, Anno, #lint{usage = Usage, file = File} = St) ->
     Used = Usage#usage.used_types,
-    NewUsed = maps_prepend(TypePair, erl_anno:set_file(File, Anno), Used),
+    UsedType = #used_type{anno = erl_anno:set_file(File, Anno),
+                          at = St#lint.type_id},
+    NewUsed = maps_prepend(TypePair, UsedType, Used),
     St#lint{usage=Usage#usage{used_types=NewUsed}}.
 
 is_default_type({Name, NumberOfTypeVariables}) ->
@@ -3120,12 +3136,12 @@ spec_decl(Anno, MFA0, TypeSpecs, St00 = #lint{specs = Specs, module = Mod}) ->
     case is_map_key(MFA, Specs) of
 	true -> add_error(Anno, {redefine_spec, MFA0}, St1);
 	false ->
-            case MFA of
-                {Mod, _, _} ->
-                    check_specs(TypeSpecs, spec_wrong_arity, Arity, St1);
-                _ ->
-                    add_error(Anno, {bad_module, MFA}, St1)
-            end
+            St2 = case MFA of
+                      {Mod, _, _} -> St1;
+                      _ -> add_error(Anno, {bad_module, MFA}, St1)
+                  end,
+            St3 = St2#lint{type_id = {spec, MFA}},
+            check_specs(TypeSpecs, spec_wrong_arity, Arity, St3)
     end.
 
 %% callback_decl(Anno, Fun, Types, State) -> State.
@@ -3141,8 +3157,9 @@ callback_decl(Anno, MFA0, TypeSpecs,
             St1 = St0#lint{callbacks = maps:put(MFA, Anno, Callbacks)},
             case is_map_key(MFA, Callbacks) of
                 true -> add_error(Anno, {redefine_callback, MFA0}, St1);
-                false -> check_specs(TypeSpecs, callback_wrong_arity,
-                                     Arity, St1)
+                false ->
+                    St2 = St1#lint{type_id = {spec, MFA}},
+                    check_specs(TypeSpecs, callback_wrong_arity, Arity, St2)
             end
     end.
 
@@ -3263,11 +3280,10 @@ check_unused_types(Forms, St) ->
         false -> St
     end.
 
-check_unused_types_1(Forms, #lint{usage=Usage, types=Ts, exp_types=ExpTs}=St) ->
+check_unused_types_1(Forms, #lint{types=Ts}=St) ->
     case [File || {attribute,_A,file,{File,_Anno}} <- Forms] of
 	[FirstFile|_] ->
-	    D = Usage#usage.used_types,
-	    L = gb_sets:to_list(ExpTs) ++ maps:keys(D),
+            L = reached_types(St),
 	    UsedTypes = gb_sets:from_list(L),
 	    FoldFun =
                 fun({{record, _}=_Type, 0}, _, AccSt) ->
@@ -3291,6 +3307,19 @@ check_unused_types_1(Forms, #lint{usage=Usage, types=Ts, exp_types=ExpTs}=St) ->
 	    St
     end.
 
+reached_types(#lint{usage = Usage}) ->
+    Es = [{From, {type, To}} ||
+             {To, UsedTs} <- maps:to_list(Usage#usage.used_types),
+             #used_type{at = From} <- UsedTs],
+    Initial = initially_reached_types(Es),
+    G = sofs:family_to_digraph(sofs:rel2fam(sofs:relation(Es))),
+    R = digraph_utils:reachable(Initial, G),
+    true = digraph:delete(G),
+    [T || {type, T} <- R].
+
+initially_reached_types(Es) ->
+    [FromTypeId || {{T, _}=FromTypeId, _} <- Es, T =/= type].
+
 check_local_opaque_types(St) ->
     #lint{types=Ts, exp_types=ExpTs} = St,
     FoldFun =