Merge pull request #2501 from dotty-staging/add-phantom-doc

Add note on phantom universe mismatch

Author: felixmulder

compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala

@@ -120,6 +120,8 @@ object ErrorReporting {
       val expected1 = dropJavaMethod(expected)
       if ((found1 eq found) != (expected eq expected1) && (found1 <:< expected1))
         "\n (Note that Scala's and Java's representation of this type differs)"
+      else if (found.topType != expected.topType)
+        "\n (Note that the types are in different universes, see Phantom types)"
       else if (ctx.settings.explainTypes.value)
         "\n" + ctx.typerState.show + "\n" + TypeComparer.explained((found <:< expected)(_))
       else