"cannot infer type; expected type <?> is not fully defined" for structural calls

[sjrd]

Minimized code

import scala.reflect.Selectable.reflectiveSelectable

object Test {
  def main(args: Array[String]): Unit = {
    class Foo {
      def makeInt: Int = 5
      def testInt(x: Int): Unit = assert(5 == x)

      def makeRef: Option[String] = Some("hi")
      def testRef(x: Option[String]): Unit = assert(Some("hi") == x)
    }

    def test(foo: {
      def makeInt: Int
      def testInt(x: Int): Unit
      def makeRef: Option[String]
      def testRef(x: Option[String]): Unit
    }): Unit = {
      foo.testInt(foo.makeInt)
               //            ^
               // cannot infer type; expected type <?> is not fully defined
      foo.testRef(foo.makeRef)
               //            ^
               // cannot infer type; expected type <?> is not fully defined
    }

    test(new Foo)
  }
}

Output

-- Error: tests/run/hello.scala:24:29 ------------------------------------------
24 |      foo.testInt(foo.makeInt)
   |                             ^
   |               cannot infer type; expected type <?> is not fully defined
1 error found

(if we comment that line, the same error appears on the next line)

Expectation

All types are completely defined here, so I expect that there's nothing to be inferred, and therefore the "cannot infer type" error message doesn't make sense to me. I expect this code to compile and successfully run.