From 5ff36749dd6953dd989bdc7b4912d5248e671807 Mon Sep 17 00:00:00 2001
From: Radek Szymczyszyn <radoslaw.szymczyszyn@erlang-solutions.com>
Date: Wed, 7 Apr 2021 12:14:57 +0200
Subject: Suggest {preprocess, true} when redundant spec assertion fails

---
 lib/edoc/src/edoc.erl         |  3 ++-
 lib/edoc/src/edoc.hrl         |  3 ++-
 lib/edoc/src/edoc_extract.erl | 15 +++++++++++++--
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/lib/edoc/src/edoc.erl b/lib/edoc/src/edoc.erl
index a811037b5f..6ec5565fb0 100644
--- a/lib/edoc/src/edoc.erl
+++ b/lib/edoc/src/edoc.erl
@@ -77,7 +77,8 @@
 			       exports :: ordset(function_name()),
 			       attributes :: ordset({atom(), term()}),
 			       records :: [{atom(), [{atom(), term()}]}],
-			       encoding :: epp:source_encoding()}.
+			       encoding :: epp:source_encoding(),
+			       file :: file:filename()}.
 %% Module information.
 
 -type env() :: #env{}.
diff --git a/lib/edoc/src/edoc.hrl b/lib/edoc/src/edoc.hrl
index 9365d2ea5b..82d1fe932f 100644
--- a/lib/edoc/src/edoc.hrl
+++ b/lib/edoc/src/edoc.hrl
@@ -50,7 +50,8 @@
 		 exports = [],
 		 attributes = [],
 		 records = [],
-		 encoding = latin1}).
+		 encoding = latin1,
+		 file}).
 
 -record(env, {module = [],
 	      root = "",
diff --git a/lib/edoc/src/edoc_extract.erl b/lib/edoc/src/edoc_extract.erl
index 75df6637ab..cef5e55703 100644
--- a/lib/edoc/src/edoc_extract.erl
+++ b/lib/edoc/src/edoc_extract.erl
@@ -328,7 +328,8 @@ get_module_info(Forms, File) ->
 	    exports = ordsets:intersection(Exports, Functions),
 	    attributes = Attributes,
 	    records = Records,
-	    encoding = Encoding}.
+	    encoding = Encoding,
+	    file = File}.
 
 get_list_keyval(Key, L) ->
     case lists:keyfind(Key, 1, L) of
@@ -483,9 +484,19 @@ insert_specs(As, Ss, Mod) ->
     Specs = maps:from_list(SpecList),
     %% Assert that we've not skipped redundant specs for the same {Fun, Arity}.
     %% This should never happen, as such a module would not compile.
-    true = length(SpecList) == maps:size(Specs),
+    case length(SpecList) == maps:size(Specs) of
+	true -> ok;
+	false -> error_redundant_specs(Mod, SpecList, Specs)
+    end,
     insert_specs_(ModName, As, Specs).
 
+error_redundant_specs(Mod, SpecList, Specs) ->
+    [{RedundantMFA, [Form]} | _] = lists:sort(SpecList) -- lists:sort(maps:to_list(Specs)),
+    {_, Line, _, _} = erl_syntax:revert(Form),
+    {_, F, A} = RedundantMFA,
+    edoc_report:error(Line, {Mod#module.file, {F, A}}, "Redundant -spec attribute found. Try setting {preprocess, true}."),
+    erlang:exit({redundant_spec, RedundantMFA}).
+
 insert_specs_(_, [], _) -> [];
 insert_specs_(ModName, [#entry{} = A | As], Specs) ->
     #entry{name = {F, Arity}, data = {Cs, Cbs, _, Ts, Rs}} = A,
-- 
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