Correctly shrink constrained types with sampleshrink

Without this patch shrinking with sampleshrink takes forever
for some types when retries to this function retries to shrink.
The solution is to compare shrunk result with previous results
and stop if we already saw the value

src/proper_gen.erl

@@ -276,7 +276,14 @@ keep_shrinking(ImmInstance, Acc, Type, State) ->
 	    %% try next shrinker
 	    keep_shrinking(ImmInstance, Acc, Type, NewState);
 	{[Shrunk|_Rest], _NewState} ->
-	    keep_shrinking(Shrunk, [ImmInstance|Acc], Type)
+        Acc2 = [ImmInstance|Acc],
+        case lists:member(Shrunk, Acc2) of
+            true ->
+                %% Avoid infinite loops
+                lists:reverse(Acc2);
+            false ->
+                keep_shrinking(Shrunk, Acc2, Type)
+        end
     end.
 
 -spec contains_fun(term()) -> boolean().

test/proper_tests.erl

@@ -1168,6 +1168,17 @@ dollar_only_cp_test_() ->
 	     re:run(atom_to_list(K), ["^[$]"], [{capture,none}]) =:= match]).
 
 
+sampleshrink_test_() ->
+    [{"Test type with restrain",
+      [{"Try another way to call shrinking (not sampleshrink)",
+        ?_shrinksTo([a], non_empty(?LET({Len,List},
+                                        {range(0,5), list(a)},
+                                        lists:sublist(List, Len))))},
+       ?_test(proper_gen:sampleshrink(non_empty(?LET({Len,List},
+                                       {range(0,5), list(a)},
+                                       lists:sublist(List, Len)))))]}].
+
+
 %%------------------------------------------------------------------------------
 %% Performance tests
 %%------------------------------------------------------------------------------