Fix regression on grammar, closes #12579

Author: josevalim

lib/elixir/src/elixir_parser.yrl

@@ -153,6 +153,7 @@ matched_expr -> access_expr kw_identifier : error_invalid_kw_identifier('$2').
 unmatched_expr -> matched_expr unmatched_op_expr : build_op('$1', '$2').
 unmatched_expr -> unmatched_expr matched_op_expr : build_op('$1', '$2').
 unmatched_expr -> unmatched_expr unmatched_op_expr : build_op('$1', '$2').
+unmatched_expr -> unmatched_expr no_parens_op_expr : warn_no_parens_after_do_op('$2'), build_op('$1', '$2').
 unmatched_expr -> unary_op_eol expr : build_unary_op('$1', '$2').
 unmatched_expr -> at_op_eol expr : build_unary_op('$1', '$2').
 unmatched_expr -> capture_op_eol expr : build_unary_op('$1', '$2').
@@ -1218,6 +1219,16 @@ warn_pipe({arrow_op, {Line, Column, _}, Op}, {_, [_ | _], [_ | _]}) ->
 warn_pipe(_Token, _) ->
   ok.
 
+%% TODO: Make this an error on v2.0
+warn_no_parens_after_do_op({{_Type, Location, Op}, _}) ->
+  {Line, _, _} = Location,
+
+  warn(
+    Line,
+    "missing parentheses on expression following operator \"" ++ atom_to_list(Op) ++ "\", "
+    "you must add parentheses to avoid ambiguities"
+  ).
+
 %% TODO: Make this an error on v2.0
 warn_nested_no_parens_keyword(Key, Value) when is_atom(Key) ->
   {line, Line} = lists:keyfind(line, 1, ?meta(Value)),

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

@@ -1647,6 +1647,17 @@ defmodule Kernel.WarningTest do
            end) =~ "missing parentheses for expression following \"label:\" keyword. "
   end
 
+  test "do+end with operator without explicit parentheses" do
+    assert capture_err(fn ->
+             Code.eval_string("""
+             quote do
+               case do
+               end || raise 1, 2
+             end
+             """)
+           end) =~ "missing parentheses on expression following operator \"||\""
+  end
+
   test "variable is being expanded to function call (on_undefined_variable: warn)" do
     Code.put_compiler_option(:on_undefined_variable, :warn)