Raise on middle expression without start expression in EEx (elixir-la…

…ng#6610)

lib/eex/lib/eex/compiler.ex

@@ -53,7 +53,7 @@ defmodule EEx.Compiler do
     generate_buffer(rest, state.engine.handle_begin(buffer), [wrapped | scope], %{state | line: line})
   end
 
-  defp generate_buffer([{:middle_expr, line, modifier, chars} | t], buffer, scope, state) do
+  defp generate_buffer([{:middle_expr, line, modifier, chars} | t], buffer, [_ | _] = scope, state) do
     message = "unexpected beginning of EEx tag \"<%#{modifier}\" on \"<%#{modifier}#{chars}%>\", " <>
               "please remove \"#{modifier}\" accordingly"
     :elixir_errors.warn line, state.file, message
@@ -62,6 +62,11 @@ defmodule EEx.Compiler do
     # raise EEx.SyntaxError, message: message, file: state.file, line: line
   end
 
+  defp generate_buffer([{:middle_expr, line, _, chars} | _], _buffer, [], state) do
+    raise EEx.SyntaxError, message: "unexpected middle of expression <%#{chars}%>",
+                           file: state.file, line: line
+  end
+
   defp generate_buffer([{:end_expr, line, '', chars} | rest], buffer, [current | _], state) do
     {wrapped, state} = wrap_expr(current, line, buffer, chars, state)
     tuples = Code.string_to_quoted!(wrapped, [line: state.start_line, file: state.file])

lib/eex/test/eex_test.exs

@@ -150,6 +150,12 @@ defmodule EExTest do
       end
     end
 
+    test "when middle expression is found without a start expression" do
+      assert_raise EEx.SyntaxError, "nofile:1: unexpected middle of expression <% else %>", fn ->
+        EEx.compile_string "<% if true %> foo<% else %>bar<% end %>"
+      end
+    end
+
     test "when end expression is found without a start expression" do
       assert_raise EEx.SyntaxError, "nofile:1: unexpected end of expression <% end %>",  fn ->
         EEx.compile_string "foo <% end %>"