Add error message for polymorphic methods in structured refinements

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -124,6 +124,7 @@ public enum ErrorMessageID {
     SymbolChangedSemanticsInVersionID,
     UnableToEmitSwitchID,
     MissingCompanionForStaticID,
+    PolymorphicMethodMissingTypeInParentID,
     ;
 
     public int errorNumber() {

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -2064,4 +2064,14 @@ object messages {
     val explanation =
       hl"An object that contains ${"@static"} members must have a companion class."
   }
+
+  case class PolymorphicMethodMissingTypeInParent(rsym: Symbol, parentTpt: tpd.Tree)(implicit ctx: Context)
+  extends Message(PolymorphicMethodMissingTypeInParentID) {
+    val kind = "Syntax"
+    val msg = hl"polymorphic refinement $rsym without matching type in parent $parentTpt is no longer allowed"
+    val explanation =
+      hl"""Polymorphic $rsym is not allowed in the structural refinement of $parentTpt because
+          |$rsym does not override any symbol in $parentTpt. Structural refinement does not allow for
+          |polymorphic methods."""
+  }
 }

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

@@ -1169,7 +1169,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       checkRefinementNonCyclic(refinement, refineCls, seen)
       val rsym = refinement.symbol
       if (rsym.info.isInstanceOf[PolyType] && rsym.allOverriddenSymbols.isEmpty)
-        ctx.error(i"polymorphic refinement $rsym without matching type in parent $tpt1 is no longer allowed", refinement.pos)    }
+        ctx.error(PolymorphicMethodMissingTypeInParent(rsym, tpt1), refinement.pos)    }
     assignType(cpy.RefinedTypeTree(tree)(tpt1, refinements1), tpt1, refinements1, refineCls)
   }
 

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1277,4 +1277,21 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val MissingCompanionForStatic(member) = messages.head
       assertEquals(member.show, "method bar")
     }
+
+  @Test def polymorphicMethodMissingTypeInParent =
+    checkMessagesAfter("frontend") {
+      """
+        |object Test {
+        |  import scala.reflect.Selectable.reflectiveSelectable
+        |  def foo(x: { def get[T](a: T): Int }) = 5
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+      val PolymorphicMethodMissingTypeInParent(rsym, parentTpt) = messages.head
+      assertEquals("method get", rsym.show)
+      assertEquals("Object", parentTpt.show)
+    }
 }