Emit warnings for duplicate patterns instead of errors

Author: josevalim

lib/elixir/src/elixir_clauses.erl

@@ -11,7 +11,7 @@
 
 parallel_match(Meta, Expr, S, #{context := match} = E) ->
   #elixir_ex{vars={_Read, Write}} = S,
-  Matches = unpack_match(Expr, Meta, []),
+  Matches = unpack_match(Expr, Meta, [], E),
 
   {[{_, EHead} | ETail], EWrites, SM, EM} =
     lists:foldl(fun({EMeta, Match}, {AccMatches, AccWrites, SI, EI}) ->
@@ -31,9 +31,13 @@ parallel_match(Meta, Expr, S, #{context := match} = E) ->
   VWrite = (Write /= false) andalso elixir_env:merge_vars(Write, PWrites),
   {EMatch, SM#elixir_ex{vars={VRead, VWrite}, prematch={PRead, PCycles, PInfo}}, EM}.
 
-unpack_match({'=', Meta, [Left, Right]}, _Meta, Acc) ->
-  unpack_match(Left, Meta, unpack_match(Right, Meta, Acc));
-unpack_match(Node, Meta, Acc) ->
+unpack_match({'=', Meta, [{_, VarMeta, _} = Node, Node]}, _Meta, Acc, E) ->
+  %% TODO: remove this clause on Elixir v1.23
+  file_warn(VarMeta, ?key(E, file), ?MODULE, {duplicate_match, Node}),
+  unpack_match(Node, Meta, Acc, E);
+unpack_match({'=', Meta, [Left, Right]}, _Meta, Acc, E) ->
+  unpack_match(Left, Meta, unpack_match(Right, Meta, Acc, E), E);
+unpack_match(Node, Meta, Acc, _E) ->
   [{Meta, Node} | Acc].
 
 store_cycles([Write | Writes], {Cycles, SkipList}, Acc) ->
@@ -532,6 +536,13 @@ origin(Meta, Default) ->
     false -> Default
   end.
 
+format_error({duplicate_match, Expr}) ->
+  String = 'Elixir.Macro':to_string(Expr),
+  io_lib:format(
+    "this pattern is matched against itself inside a match: ~ts = ~ts",
+    [String, String]
+  );
+
 format_error({recursive, Vars, TypeExpr}) ->
   Code =
     case TypeExpr of

lib/elixir/test/elixir/kernel/expansion_test.exs

@@ -190,17 +190,6 @@ defmodule Kernel.ExpansionTest do
     end
 
     test "errors on directly recursive definitions" do
-      assert_compile_error(
-        ~r"""
-        recursive variable definition in patterns:
-
-        x = x
-
-        the variable "x" \(context Kernel.ExpansionTest\) is defined in function of itself
-        """,
-        fn -> expand(quote(do: (x = x) = :ok)) end
-      )
-
       assert_compile_error(
         ~r"""
         recursive variable definition in patterns:

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -300,6 +300,20 @@ defmodule Kernel.WarningTest do
     purge(Sample)
   end
 
+  test "duplicate pattern" do
+    output =
+      capture_eval("""
+      defmodule Sample do
+        var = quote(do: x)
+        def hello(unquote(var) = unquote(var)), do: unquote(var)
+      end
+      """)
+
+    assert output =~ "this pattern is matched against itself inside a match: x = x"
+  after
+    purge(Sample)
+  end
+
   test "unused compiler variable" do
     output =
       capture_eval("""