Raise CompileError in case of duplicated do clauses for case

Author: lexmag

lib/elixir/src/elixir_exp_clauses.erl

@@ -47,6 +47,9 @@ guard(Other, E) ->
 'case'(Meta, KV, E) when not is_list(KV) ->
   compile_error(Meta, ?m(E, file), "invalid arguments for case");
 'case'(Meta, KV, E) ->
+  ok = assert_at_most_once('do', KV, 0, fun(Kind) ->
+    compile_error(Meta, ?m(E, file), "duplicated ~ts clauses given for case", [Kind])
+  end),
   EE = E#{export_vars := []},
   {EClauses, EVars} = lists:mapfoldl(fun(X, Acc) -> do_case(Meta, X, Acc, EE) end, [], KV),
   {EClauses, elixir_env:mergev(EVars, E)}.

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

@@ -359,6 +359,12 @@ defmodule Kernel.ExpansionTest do
            quote do: (case w() do x -> x = x; y -> y = y end; :erlang.+(x, y))
   end
 
+  test "case: expects at most one do" do
+    assert_raise CompileError, ~r"duplicated do clauses given for case", fn ->
+      expand(quote(do: (case e, do: (x -> x), do: (y -> y))))
+    end
+  end
+
   ## Receive
 
   test "receive: expands each clause" do