Improve overriding error message for exports

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

@@ -362,6 +362,8 @@ object RefChecks {
             s"$clazz inherits conflicting members:\n  "
               + infoStringWithLocation(other) + "  and\n  " + infoStringWithLocation(member)
               + "\n(Note: this can be resolved by declaring an override in " + clazz + ".)")
+        else if member.is(Exported) then
+          overrideError("cannot override since it comes from an export")
         else
           overrideError("needs `override` modifier")
       else if (other.is(AbsOverride) && other.isIncompleteIn(clazz) && !member.is(AbsOverride))

tests/neg/exports1.scala

@@ -0,0 +1,20 @@
+object A:
+  def f: String = ""
+
+trait B:
+  def f: String = "abc"
+
+trait B2 extends B:
+  override def f: String = "abc"
+
+object D extends B:
+  object b extends B
+  export b._             // ok
+
+object D1 extends B:
+  object b extends B
+  export b.f             // error
+
+object D2 extends B:
+  object b2 extends B2
+  export b2.f            // error

tests/neg/exports2.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/exports2.scala:8:11 --------------------------------------------------------------------------------
+8 |  export A._            // error
+  |           ^
+  |           error overriding method f in trait B of type => String;
+  |             method f of type => String cannot override since it comes from an export

tests/neg/exports2.scala

@@ -0,0 +1,9 @@
+object A:
+  def f: String = ""
+
+trait B:
+  def f: String = "abc"
+
+object C extends B:
+  export A._            // error
+