name profile directories in `_build` similarly to the arguments

to `as` used to generate them

src/rebar_dir.erl

@@ -23,7 +23,12 @@
 -spec base_dir(rebar_state:t()) -> file:filename_all().
 base_dir(State) ->
     Profiles = rebar_state:current_profiles(State),
-    ProfilesStrings = [ec_cnv:to_list(P) || P <- Profiles],
+    ProfilesStrings = case [ec_cnv:to_list(P) || P <- Profiles] of
+        ["default"]      -> ["default"];
+        %% drop `default` from the profile dir if it's implicit and reverse order
+        %%  of profiles to match order passed to `as`
+        ["default"|Rest] -> lists:reverse(Rest)
+    end,
     ProfilesDir = string:join(ProfilesStrings, "+"),
     filename:join(rebar_state:get(State, base_dir, ?DEFAULT_BASE_DIR), ProfilesDir).
 

test/rebar_as_SUITE.erl

@@ -9,7 +9,8 @@
          as_multiple_profiles/1,
          as_multiple_tasks/1,
          as_multiple_profiles_multiple_tasks/1,
-         as_comma_placement/1]).
+         as_comma_placement/1,
+         as_dir_name/1]).
 
 -include_lib("common_test/include/ct.hrl").
 -include_lib("eunit/include/eunit.hrl").
@@ -25,7 +26,8 @@ init_per_testcase(_, Config) ->
     rebar_test_utils:init_rebar_state(Config, "as_").
 
 all() -> [as_basic, as_multiple_profiles, as_multiple_tasks,
-          as_multiple_profiles_multiple_tasks, as_comma_placement].
+          as_multiple_profiles_multiple_tasks, as_comma_placement,
+          as_dir_name].
 
 as_basic(Config) ->
     AppDir = ?config(apps, Config),
@@ -87,3 +89,17 @@ as_comma_placement(Config) ->
                                    ["as", "foo,bar", ",", "baz", ",qux", "compile"],
                                    {ok, [{app, Name}]}).
 
+as_dir_name(Config) ->
+    AppDir = ?config(apps, Config),
+
+    Name = rebar_test_utils:create_random_name("as_dir_name_"),
+    Vsn = rebar_test_utils:create_random_vsn(),
+    rebar_test_utils:create_app(AppDir, Name, Vsn, [kernel, stdlib]),
+
+    rebar_test_utils:run_and_check(Config,
+                                   [],
+                                   ["as", "foo,bar,baz", "compile"],
+                                   {ok, [{app, Name}]}),
+
+    true = filelib:is_dir(filename:join([AppDir, "_build", "foo+bar+baz"])).
+