Generate suggested stub methods as seen from implementing class

Fixes #13076

Co-authored-by: Seth Tisue <seth@tisue.net>

Author: griggt

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

@@ -589,7 +589,7 @@ object RefChecks {
           // Grouping missing methods by the declaring class
           val regrouped = missingMethods.groupBy(_.owner).toList
           def membersStrings(members: List[Symbol]) =
-            members.sortBy(_.name.toString).map(_.showDcl + " = ???")
+            members.sortBy(_.name.toString).map(_.asSeenFrom(clazz.thisType).showDcl + " = ???")
 
           if (regrouped.tail.isEmpty)
             membersStrings(regrouped.head._2)

tests/neg/i13076.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/i13076.scala:6:6 -----------------------------------------------------------------------------------
+6 |class C[X, Y] extends T[X, Y] {} // error
+  |      ^
+  |      class C needs to be abstract, since:
+  |      it has 2 unimplemented members.
+  |      /** As seen from class C, the missing signatures are as follows.
+  |       *  For convenience, these are usable as stub implementations.
+  |       */
+  |        def f(a: X): Y = ???
+  |        def g(a: X, b: Y): Unit = ???

tests/neg/i13076.scala

@@ -0,0 +1,6 @@
+trait T[A, B] {
+  def f(a: A): B
+  def g(a: A, b: B): Unit
+}
+
+class C[X, Y] extends T[X, Y] {} // error