8 files changed, 222 insertions, 76 deletions

diff --git a/lib/dialyzer/src/dialyzer.erl b/lib/dialyzer/src/dialyzer.erl
index b4161ea194..471f9fccd2 100644
--- a/lib/dialyzer/src/dialyzer.erl
+++ b/lib/dialyzer/src/dialyzer.erl
@@ -106,27 +106,35 @@ cl_print_plt_info(Opts) ->
       end,
   doit(F).
 
-print_plt_info(#options{init_plt = PLT, output_file = OutputFile}) ->
+print_plt_info(#options{init_plts = PLTs, output_file = OutputFile}) ->
+  PLTInfo = get_plt_info(PLTs),
+  do_print_plt_info(PLTInfo, OutputFile).
+
+get_plt_info([PLT|PLTs]) ->
   String =
     case dialyzer_plt:included_files(PLT) of
       {ok, Files} ->
-	io_lib:format("The PLT ~s includes the following files:\n~p\n",
+	io_lib:format("The PLT ~s includes the following files:\n~p\n\n",
 		      [PLT, Files]);
       {error, read_error} ->
-	Msg = io_lib:format("Could not read the PLT file ~p\n", [PLT]),
+	Msg = io_lib:format("Could not read the PLT file ~p\n\n", [PLT]),
 	throw({dialyzer_error, Msg});
       {error, no_such_file} ->
-	Msg = io_lib:format("The PLT file ~p does not exist\n", [PLT]),
+	Msg = io_lib:format("The PLT file ~p does not exist\n\n", [PLT]),
 	throw({dialyzer_error, Msg})
     end,
+  String ++ get_plt_info(PLTs);
+get_plt_info([]) -> "".
+
+do_print_plt_info(PLTInfo, OutputFile) ->
   case OutputFile =:= none of
     true ->
-      io:format("~s", [String]),
+      io:format("~s", [PLTInfo]),
       ?RET_NOTHING_SUSPICIOUS;
     false ->
       case file:open(OutputFile, [write]) of
 	{ok, FileDesc} ->
-	  io:format(FileDesc, "~s", [String]),
+	  io:format(FileDesc, "~s", [PLTInfo]),
 	  ok = file:close(FileDesc),
 	  ?RET_NOTHING_SUSPICIOUS;
 	{error, Reason} ->
diff --git a/lib/dialyzer/src/dialyzer.hrl b/lib/dialyzer/src/dialyzer.hrl
index 2da8ed2e5d..1d98574585 100644
--- a/lib/dialyzer/src/dialyzer.hrl
+++ b/lib/dialyzer/src/dialyzer.hrl
@@ -129,7 +129,7 @@
 		  defines         = []		   :: [dial_define()],
 		  from            = byte_code	   :: start_from(),
 		  get_warnings    = maybe          :: boolean() | 'maybe',
-		  init_plt        = none	   :: 'none' | file:filename(),
+		  init_plts       = []	           :: [file:filename()],
 		  include_dirs    = []		   :: [file:filename()],
 		  output_plt      = none           :: 'none' | file:filename(),
 		  legal_warnings  = ordsets:new()  :: ordset(dial_warn_tag()),
diff --git a/lib/dialyzer/src/dialyzer_cl.erl b/lib/dialyzer/src/dialyzer_cl.erl
index 0250c47ad0..1987c1732c 100644
--- a/lib/dialyzer/src/dialyzer_cl.erl
+++ b/lib/dialyzer/src/dialyzer_cl.erl
@@ -81,11 +81,15 @@ build_plt(Opts) ->
 init_opts_for_build(Opts) ->
   case Opts#options.output_plt =:= none of
     true ->
-      case Opts#options.init_plt of
-	none -> Opts#options{init_plt = none, output_plt = get_default_plt()};
-	Plt  -> Opts#options{init_plt = none, output_plt = Plt}
+      case Opts#options.init_plts of
+	[] -> Opts#options{output_plt = get_default_output_plt()};
+	[Plt] -> Opts#options{init_plts = [], output_plt = Plt};
+        Plts ->
+          Msg = io_lib:format("Could not build multiple PLT files: ~s\n",
+                              [format_plts(Plts)]),
+          error(Msg)
       end;
-    false -> Opts#options{init_plt = none}
+    false -> Opts#options{init_plts = []}
   end.
 
 %%--------------------------------------------------------------------
@@ -98,39 +102,58 @@ add_to_plt(Opts) ->
 init_opts_for_add(Opts) ->
   case Opts#options.output_plt =:= none of
     true ->
-      case Opts#options.init_plt of
-	none -> Opts#options{output_plt = get_default_plt(),
-			     init_plt = get_default_plt()};
-	Plt  -> Opts#options{output_plt = Plt}
+      case Opts#options.init_plts of
+	[] -> Opts#options{output_plt = get_default_output_plt(),
+                           init_plts = get_default_init_plt()};
+	[Plt] -> Opts#options{output_plt = Plt};
+        Plts ->
+          Msg = io_lib:format("Could not add to multiple PLT files: ~s\n",
+                              [format_plts(Plts)]),
+          error(Msg)
       end;
     false ->
-      case Opts#options.init_plt =:= none of
-	true  -> Opts#options{init_plt = get_default_plt()};
+      case Opts#options.init_plts =:= [] of
+	true  -> Opts#options{init_plts = get_default_init_plt()};
 	false -> Opts
       end
   end.
 
 %%--------------------------------------------------------------------
 
-check_plt(Opts) ->
+check_plt(#options{init_plts = []} = Opts) ->
   Opts1 = init_opts_for_check(Opts),
-  report_check(Opts),
-  plt_common(Opts1, [], []).
+  report_check(Opts1),
+  plt_common(Opts1, [], []);
+check_plt(#options{init_plts = Plts} = Opts) ->
+  check_plt_aux(Plts, Opts).
+
+check_plt_aux([_] = Plt, Opts) ->
+  Opts1 = Opts#options{init_plts = Plt},
+  Opts2 = init_opts_for_check(Opts1),
+  report_check(Opts2),
+  plt_common(Opts2, [], []);
+check_plt_aux([Plt|Plts], Opts) ->
+  Opts1 = Opts#options{init_plts = [Plt]},
+  Opts2 = init_opts_for_check(Opts1),
+  report_check(Opts2),
+  plt_common(Opts2, [], []),
+  check_plt_aux(Plts, Opts).
 
 init_opts_for_check(Opts) ->
-  Plt =
-    case Opts#options.init_plt of
-      none -> get_default_plt();
-      Plt0 -> Plt0
+  InitPlt =
+    case Opts#options.init_plts of
+      []-> get_default_init_plt();
+      Plt -> Plt
     end,
+  [OutputPlt] = InitPlt,
   Opts#options{files         = [],
 	       files_rec     = [],
 	       analysis_type = plt_check,
 	       defines       = [],
 	       from          = byte_code,
-	       init_plt      = Plt,
+	       init_plts     = InitPlt,
 	       include_dirs  = [],
-	       output_plt    = Plt,
+	       output_plt    = OutputPlt,
 	       use_contracts = true
 	      }.
 
@@ -144,21 +167,25 @@ remove_from_plt(Opts) ->
 init_opts_for_remove(Opts) ->
   case Opts#options.output_plt =:= none of
     true ->
-      case Opts#options.init_plt of
-	none -> Opts#options{output_plt = get_default_plt(),
-			     init_plt = get_default_plt()};
-	Plt  -> Opts#options{output_plt = Plt}
+      case Opts#options.init_plts of
+	[] -> Opts#options{output_plt = get_default_output_plt(),
+                           init_plts = get_default_init_plt()};
+	[Plt] -> Opts#options{output_plt = Plt};
+        Plts ->
+          Msg = io_lib:format("Could not remove from multiple PLT files: ~s\n",
+                              [format_plts(Plts)]),
+          error(Msg)
       end;
     false ->
-      case Opts#options.init_plt =:= none of
-	true  -> Opts#options{init_plt = get_default_plt()};
+      case Opts#options.init_plts =:= [] of
+	true  -> Opts#options{init_plts = get_default_init_plt()};
 	false -> Opts
       end
   end.
 
 %%--------------------------------------------------------------------
 
-plt_common(Opts, RemoveFiles, AddFiles) ->
+plt_common(#options{init_plts = [InitPlt]} = Opts, RemoveFiles, AddFiles) ->
   case check_plt(Opts, RemoveFiles, AddFiles) of
     ok ->
       case Opts#options.report_mode of
@@ -174,7 +201,7 @@ plt_common(Opts, RemoveFiles, AddFiles) ->
       report_failed_plt_check(Opts, DiffMd5),
       {AnalFiles, RemovedMods, ModDeps1} = 
 	expand_dependent_modules(Md5, DiffMd5, ModDeps),
-      Plt = clean_plt(Opts#options.init_plt, RemovedMods),
+      Plt = clean_plt(InitPlt, RemovedMods),
       case AnalFiles =:= [] of
 	true ->
 	  %% Only removed stuff. Just write the PLT.
@@ -186,19 +213,19 @@ plt_common(Opts, RemoveFiles, AddFiles) ->
       end;
     {error, no_such_file} ->
       Msg = io_lib:format("Could not find the PLT: ~s\n~s",
-			  [Opts#options.init_plt, default_plt_error_msg()]),
+			  [InitPlt, default_plt_error_msg()]),
       error(Msg);
     {error, not_valid} ->
       Msg = io_lib:format("The file: ~s is not a valid PLT file\n~s",
-			  [Opts#options.init_plt, default_plt_error_msg()]),
+			  [InitPlt, default_plt_error_msg()]),
       error(Msg);
     {error, read_error} ->
       Msg = io_lib:format("Could not read the PLT: ~s\n~s",
-			  [Opts#options.init_plt, default_plt_error_msg()]),
+			  [InitPlt, default_plt_error_msg()]),
       error(Msg);
     {error, {no_file_to_remove, F}} ->
       Msg = io_lib:format("Could not remove the file ~s from the PLT: ~s\n",
-			  [F, Opts#options.init_plt]),
+			  [F, InitPlt]),
       error(Msg)
   end.
 
@@ -218,8 +245,7 @@ default_plt_error_msg() ->
 
 %%--------------------------------------------------------------------
 
-check_plt(Opts, RemoveFiles, AddFiles) ->
-  Plt = Opts#options.init_plt,
+check_plt(#options{init_plts = [Plt]} = Opts, RemoveFiles, AddFiles) ->
   case dialyzer_plt:check_plt(Plt, RemoveFiles, AddFiles) of
     {old_version, _MD5} = OldVersion ->
       report_old_version(Opts),
@@ -234,14 +260,14 @@ check_plt(Opts, RemoveFiles, AddFiles) ->
 
 %%--------------------------------------------------------------------
 
-report_check(#options{report_mode = ReportMode, init_plt = InitPlt}) ->
+report_check(#options{report_mode = ReportMode, init_plts = [InitPlt]}) ->
   case ReportMode of
     quiet -> ok;
     _ ->
       io:format("  Checking whether the PLT ~s is up-to-date...", [InitPlt])
   end.
 
-report_old_version(#options{report_mode = ReportMode, init_plt = InitPlt}) ->
+report_old_version(#options{report_mode = ReportMode, init_plts = [InitPlt]}) ->
   case ReportMode of
     quiet -> ok;
     _ ->
@@ -264,7 +290,7 @@ report_failed_plt_check(#options{analysis_type = AnalType,
 
 report_analysis_start(#options{analysis_type = Type,
 			       report_mode = ReportMode,
-			       init_plt = InitPlt, 
+			       init_plts = InitPlts,
 			       output_plt = OutputPlt}) ->
   case ReportMode of
     quiet -> ok;
@@ -272,6 +298,7 @@ report_analysis_start(#options{analysis_type = Type,
       io:format("  "),
       case Type of
 	plt_add ->
+          [InitPlt] = InitPlts,
 	  case InitPlt =:= OutputPlt of
 	    true -> io:format("Adding information to ~s...", [OutputPlt]);
 	    false -> io:format("Adding information from ~s to ~s...", 
@@ -282,6 +309,7 @@ report_analysis_start(#options{analysis_type = Type,
 	plt_check ->
 	  io:format("Rebuilding the information in ~s...", [OutputPlt]);
 	plt_remove ->
+          [InitPlt] = InitPlts,
 	  case InitPlt =:= OutputPlt of
 	    true -> io:format("Removing information from ~s...", [OutputPlt]);
 	    false -> io:format("Removing information from ~s to ~s...", 
@@ -320,16 +348,28 @@ report_md5_diff(List) ->
 
 %%--------------------------------------------------------------------
 
-get_default_plt() ->
+get_default_init_plt() ->
+  [dialyzer_plt:get_default_plt()].
+
+get_default_output_plt() ->
   dialyzer_plt:get_default_plt().
 
 %%--------------------------------------------------------------------
 
+format_plts([Plt]) -> Plt;
+format_plts([Plt|Plts]) ->
+  Plt ++ ", " ++ format_plts(Plts).
+
+%%--------------------------------------------------------------------
+
 do_analysis(Options) ->
   Files = get_files_from_opts(Options),
-  case Options#options.init_plt of
-    none -> do_analysis(Files, Options, dialyzer_plt:new(), none);
-    File -> do_analysis(Files, Options, dialyzer_plt:from_file(File), none)
+  case Options#options.init_plts of
+    [] -> do_analysis(Files, Options, dialyzer_plt:new(), none);
+    PltFiles ->
+      Plts = [dialyzer_plt:from_file(F) || F <- PltFiles],
+      Plt = dialyzer_plt:merge_plts_or_report_conflicts(PltFiles, Plts),
+      do_analysis(Files, Options, Plt, none)
   end.
   
 do_analysis(Files, Options, Plt, PltInfo) ->
diff --git a/lib/dialyzer/src/dialyzer_cl_parse.erl b/lib/dialyzer/src/dialyzer_cl_parse.erl
index 0160b84abc..2f9e577544 100644
--- a/lib/dialyzer/src/dialyzer_cl_parse.erl
+++ b/lib/dialyzer/src/dialyzer_cl_parse.erl
@@ -138,11 +138,17 @@ cl(["-pa", Path|T]) ->
     true -> cl(T);
     {error, _} -> error("Bad directory for -pa: "++Path)
   end;
-cl(["--plt", PLT|T]) ->
-  put(dialyzer_init_plt, PLT),
-  cl(T);
 cl(["--plt"]) ->
   error("No plt specified for --plt");
+cl(["--plt", PLT|T]) ->
+  put(dialyzer_init_plts, [PLT]),
+  cl(T);
+cl(["--plts"]) ->
+  error("No plts specified for --plts");
+cl(["--plts"|T]) ->
+  {PLTs, NewT} = get_plts(T, []),
+  put(dialyzer_init_plts, PLTs),
+  cl(NewT);
 cl(["-q"|T]) ->
   put(dialyzer_options_report_mode, quiet),
   cl(T);
@@ -284,7 +290,7 @@ common_options() ->
   [{defines, get(dialyzer_options_defines)},
    {from, get(dialyzer_options_from)},
    {include_dirs, get(dialyzer_include)},
-   {init_plt, get(dialyzer_init_plt)},
+   {plts, get(dialyzer_init_plts)},
    {output_plt, get(dialyzer_output_plt)},
    {report_mode, get(dialyzer_options_report_mode)},
    {use_spec, get(dialyzer_options_use_contracts)},
@@ -309,6 +315,13 @@ get_lib_dir([], Acc) ->
 
 %%-----------------------------------------------------------------------
 
+get_plts(["--"|T], Acc) -> {lists:reverse(Acc), T};
+get_plts(["-"++_Opt = H|T], Acc) -> {lists:reverse(Acc), [H|T]};
+get_plts([H|T], Acc) -> get_plts(T, [H|Acc]);
+get_plts([], Acc) -> {lists:reverse(Acc), []}.
+
+%%-----------------------------------------------------------------------
+
 help_warnings() ->
   S = warning_options_msg(),
   io:put_chars(S),
@@ -316,9 +329,10 @@ help_warnings() ->
 
 help_message() ->
   S = "Usage: dialyzer [--help] [--version] [--shell] [--quiet] [--verbose]
-		[-pa dir]* [--plt plt] [-Ddefine]* [-I include_dir]* 
-		[--output_plt file] [-Wwarn]* [--src] [--gui | --wx]
-		[files_or_dirs] [-r dirs] [--apps applications] [-o outfile]
+		[-pa dir]* [--plt plt] [--plts plts] [-Ddefine]*
+                [-I include_dir]* [--output_plt file] [-Wwarn]*
+                [--src] [--gui | --wx] [files_or_dirs] [-r dirs]
+                [--apps applications] [-o outfile]
 		[--build_plt] [--add_to_plt] [--remove_from_plt]
 		[--check_plt] [--no_check_plt] [--plt_info] [--get_warnings]
                 [--no_native]
@@ -362,6 +376,10 @@ Options:
   --plt plt
       Use the specified plt as the initial plt (if the plt was built 
       during setup the files will be checked for consistency)
+  --plts plts
+      Merges the specified plts to create the initial plt -- requires
+      that the plts are disjoint (i.e., do not have any module
+      appearing in more than one plt)
   -Wwarn
       A family of options which selectively turn on/off warnings
       (for help on the names of warnings use dialyzer -Whelp)
@@ -378,12 +396,12 @@ Options:
   --build_plt
       The analysis starts from an empty plt and creates a new one from the
       files specified with -c and -r. Only works for beam files.
-      Use --plt or --output_plt to override the default plt location.
+      Use --plt(s) or --output_plt to override the default plt location.
   --add_to_plt
       The plt is extended to also include the files specified with -c and -r.
-      Use --plt to specify wich plt to start from, and --output_plt to 
-      specify where to put the plt. Note that the analysis might include 
-      files from the plt if they depend on the new files. 
+      Use --plt(s) to specify wich plt to start from, and --output_plt to
+      specify where to put the plt. Note that the analysis might include
+      files from the plt if they depend on the new files.
       This option only works with beam files.
   --remove_from_plt
       The information from the files specified with -c and -r is removed
@@ -396,8 +414,8 @@ Options:
       Skip the plt check when running Dialyzer. Useful when working with
       installed plts that never change.
   --plt_info
-      Makes Dialyzer print information about the plt and then quit. The plt 
-      can be specified with --plt.
+      Makes Dialyzer print information about the plt and then quit. The plt
+      can be specified with --plt(s).
   --get_warnings
       Makes Dialyzer emit warnings even when manipulating the plt. Only 
       emits warnings for files that are actually analyzed.
diff --git a/lib/dialyzer/src/dialyzer_gui.erl b/lib/dialyzer/src/dialyzer_gui.erl
index f353638cdf..4436330f7f 100644
--- a/lib/dialyzer/src/dialyzer_gui.erl
+++ b/lib/dialyzer/src/dialyzer_gui.erl
@@ -88,8 +88,8 @@
 
 -spec start(#options{}) -> ?RET_NOTHING_SUSPICIOUS.
 
-start(DialyzerOptions = #options{from = From, init_plt = InitPltFile,
-				 legal_warnings = LegalWarnings}) ->
+start(#options{from = From, init_plts = InitPltFiles,
+               legal_warnings = LegalWarnings} = DialyzerOptions) ->
   process_flag(trap_exit, true),
 
   GS = gs:start(),
@@ -336,9 +336,13 @@ start(DialyzerOptions = #options{from = From, init_plt = InitPltFile,
   gs:config(Packer, WH),
   {ok, CWD} = file:get_cwd(),
   
-  InitPlt = try dialyzer_plt:from_file(InitPltFile)
-	    catch throw:{dialyzer_error, _} -> dialyzer_plt:new()
-	    end,
+  InitPlt =
+    case InitPltFiles of
+      [] -> dialyzer_plt:new();
+      _ ->
+        Plts = [dialyzer_plt:from_file(F) || F <- InitPltFiles],
+        dialyzer_plt:merge_plts_or_report_conflicts(InitPltFiles, Plts)
+    end,
 
   State = #gui_state{add_all = AddAll,
 		     add_file = AddFile,
diff --git a/lib/dialyzer/src/dialyzer_gui_wx.erl b/lib/dialyzer/src/dialyzer_gui_wx.erl
index 2e309d7ec1..e711c15ea7 100644
--- a/lib/dialyzer/src/dialyzer_gui_wx.erl
+++ b/lib/dialyzer/src/dialyzer_gui_wx.erl
@@ -88,7 +88,7 @@ start(DialyzerOptions) ->
   State = wx:batch(fun() -> create_window(Wx, DialyzerOptions) end),
   gui_loop(State).
 
-create_window(Wx, DialyzerOptions) ->
+create_window(Wx, #options{init_plts = InitPltFiles} = DialyzerOptions) ->
   {ok, Host} = inet:gethostname(),
 
   %%---------- initializing frame ---------
@@ -258,11 +258,15 @@ create_window(Wx, DialyzerOptions) ->
 	       plt = PltMenu,
 	       options =OptionsMenu,
 	       help = HelpMenu},
- 
-  InitPlt = try dialyzer_plt:from_file(DialyzerOptions#options.init_plt)
-	    catch throw:{dialyzer_error, _} -> dialyzer_plt:new()
-	    end,
 
+  InitPlt =
+    case InitPltFiles of
+      [] -> dialyzer_plt:new();
+      _ ->
+        Plts = [dialyzer_plt:from_file(F) || F <- InitPltFiles],
+        dialyzer_plt:merge_plts_or_report_conflicts(InitPltFiles, Plts)
+    end,
+  
   #gui_state{add = AddButton,
 	     add_dir = AddDirButton,
 	     add_rec = AddRecButton,
diff --git a/lib/dialyzer/src/dialyzer_options.erl b/lib/dialyzer/src/dialyzer_options.erl
index 010625b7bd..2c0afa6e2b 100644
--- a/lib/dialyzer/src/dialyzer_options.erl
+++ b/lib/dialyzer/src/dialyzer_options.erl
@@ -53,14 +53,21 @@ build(Opts) ->
   InitPlt = dialyzer_plt:get_default_plt(),
   DefaultOpts = #options{},
   DefaultOpts1 = DefaultOpts#options{legal_warnings = DefaultWarns1,
-				      init_plt = InitPlt},
-  try 
-    NewOpts = build_options(Opts, DefaultOpts1),
+                                     init_plts = [InitPlt]},
+  try
+    Opts1 = preprocess_opts(Opts),
+    NewOpts = build_options(Opts1, DefaultOpts1),
     postprocess_opts(NewOpts)
   catch
     throw:{dialyzer_options_error, Msg} -> {error, Msg}
   end.
 
+preprocess_opts([]) -> [];
+preprocess_opts([{init_plt, File}|Opts]) ->
+  [{plts, [File]}|preprocess_opts(Opts)];
+preprocess_opts([Opt|Opts]) ->
+  [Opt|preprocess_opts(Opts)].
+
 postprocess_opts(Opts = #options{}) ->
   Opts1 = check_output_plt(Opts),
   adapt_get_warnings(Opts1).
@@ -144,9 +151,9 @@ build_options([{OptionName, Value} = Term|Rest], Options) ->
       build_options(Rest, Options#options{from = Value});
     get_warnings ->
       build_options(Rest, Options#options{get_warnings = Value});
-    init_plt ->
-      assert_filenames([Term], [Value]),
-      build_options(Rest, Options#options{init_plt = Value});
+    plts ->
+      assert_filenames(Term, Value),
+      build_options(Rest, Options#options{init_plts = Value});
     include_dirs ->
       assert_filenames(Term, Value),
       OldVal = Options#options.include_dirs,
diff --git a/lib/dialyzer/src/dialyzer_plt.erl b/lib/dialyzer/src/dialyzer_plt.erl
index 08d0b318b5..a7ba270c41 100644
--- a/lib/dialyzer/src/dialyzer_plt.erl
+++ b/lib/dialyzer/src/dialyzer_plt.erl
@@ -51,6 +51,7 @@
 	 lookup_contract/2,
 	 lookup_module/2,
 	 merge_plts/1,
+         merge_plts_or_report_conflicts/2,
 	 new/0,
 	 plt_and_info_from_file/1,
 	 get_specs/1,
@@ -292,6 +293,38 @@ merge_plts(List) ->
        exported_types = sets_merge(ExpTypesList),
        contracts = table_merge(ContractsList)}.
 
+-spec merge_disj_plts([plt()]) -> plt().
+
+merge_disj_plts(List) ->
+  InfoList = [Info || #plt{info = Info} <- List],
+  TypesList = [Types || #plt{types = Types} <- List],
+  ExpTypesList = [ExpTypes || #plt{exported_types = ExpTypes} <- List],
+  ContractsList = [Contracts || #plt{contracts = Contracts} <- List],
+  #plt{info = table_disj_merge(InfoList),
+       types = table_disj_merge(TypesList),
+       exported_types = sets_disj_merge(ExpTypesList),
+       contracts = table_disj_merge(ContractsList)}.
+
+-spec merge_plts_or_report_conflicts([file:filename()], [plt()]) -> plt().
+
+merge_plts_or_report_conflicts(PltFiles, Plts) ->
+  try
+    merge_disj_plts(Plts)
+  catch throw:{dialyzer_error, not_disjoint_plts} ->
+      IncFiles = lists:append([begin {ok, Fs} = included_files(F), Fs end
+                               || F <- PltFiles]),
+      ConfFiles = find_duplicates(IncFiles),
+      Msg = io_lib:format("Could not merge PLTs since they are not disjoint\n"
+                          "The following files are included in more than one "
+                          "PLTs:\n~p\n", [ConfFiles]),
+      error(Msg)
+  end.
+
+find_duplicates(List) ->
+  ModList = [filename:basename(E) || E <- List],
+  SortedList = lists:usort(ModList),
+  lists:usort(ModList -- SortedList).
+  
 -spec to_file(file:filename(), plt(), mod_deps(), {[file_md5()], mod_deps()}) -> 'ok'.
 
 to_file(FileName,
@@ -556,6 +589,25 @@ table_merge([Plt|Plts], Acc) ->
   NewAcc = dict:merge(fun(_Key, Val, Val) -> Val end, Plt, Acc),
   table_merge(Plts, NewAcc).
 
+table_disj_merge([H|T]) ->
+  table_disj_merge(T, H).
+
+table_disj_merge([], Acc) ->
+  Acc;
+table_disj_merge([Plt|Plts], Acc) ->
+  case table_is_disjoint(Plt, Acc) of
+    true ->
+      NewAcc = dict:merge(fun(_Key, _Val1, _Val2) -> gazonk end,
+                          Plt, Acc),
+      table_disj_merge(Plts, NewAcc);
+    false -> throw({dialyzer_error, not_disjoint_plts})
+  end.
+
+table_is_disjoint(T1, T2) ->
+  K1 = dict:fetch_keys(T1),
+  K2 = dict:fetch_keys(T2),
+  lists:all(fun(E) -> not lists:member(E, K2) end, K1).
+
 sets_merge([H|T]) ->
   sets_merge(T, H).
 
@@ -565,6 +617,19 @@ sets_merge([Plt|Plts], Acc) ->
   NewAcc = sets:union(Plt, Acc),
   sets_merge(Plts, NewAcc).
 
+sets_disj_merge([H|T]) ->
+  sets_disj_merge(T, H).
+
+sets_disj_merge([], Acc) ->
+  Acc;
+sets_disj_merge([Plt|Plts], Acc) ->
+  case sets:is_disjoint(Plt, Acc) of
+    true ->
+      NewAcc = sets:union(Plt, Acc),
+      sets_disj_merge(Plts, NewAcc);
+    false -> throw({dialyzer_error, not_disjoint_plts})
+  end.
+
 %%---------------------------------------------------------------------------
 %% Debug utilities.