Raise CompileError in case of duplicated do clauses for cond

Author: lexmag

lib/elixir/src/elixir_exp_clauses.erl

@@ -64,6 +64,9 @@ do_case(Meta, {Key, _}, _Acc, E) ->
 'cond'(Meta, KV, E) when not is_list(KV) ->
   compile_error(Meta, ?m(E, file), "invalid arguments for cond");
 'cond'(Meta, KV, E) ->
+  ok = assert_at_most_once('do', KV, 0, fun(Kind) ->
+    compile_error(Meta, ?m(E, file), "duplicated ~ts clauses given for cond", [Kind])
+  end),
   EE = E#{export_vars := []},
   {EClauses, EVars} = lists:mapfoldl(fun(X, Acc) -> do_cond(Meta, X, Acc, EE) end, [], KV),
   {EClauses, elixir_env:mergev(EVars, E)}.

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

@@ -326,6 +326,12 @@ defmodule Kernel.ExpansionTest do
            quote do: (cond do 1 -> x = 1; 2 -> y = 2 end; :erlang.+(x, y))
   end
 
+  test "cond: expects at most one do" do
+    assert_raise CompileError, ~r"duplicated do clauses given for cond", fn ->
+      expand(quote(do: (cond do: (x -> x), do: (y -> y))))
+    end
+  end
+
   ## Case
 
   test "case: expands each clause" do