Fix #7086: Don't export overriding symbols

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

@@ -954,6 +954,12 @@ class Namer { typer: Typer =>
           else if (sym.isConstructor || sym.is(ModuleClass) || sym.is(Bridge)) SKIP
           else if (cls.derivesFrom(sym.owner) &&
                    (sym.owner == cls || !sym.is(Deferred))) i"is already a member of $cls"
+          else if (sym.is(Override))
+            sym.allOverriddenSymbols.find(
+             other => cls.derivesFrom(other.owner) && !other.is(Deferred)) match {
+               case Some(other) => i"overrides ${other.showLocated}, which is already a member of $cls"
+               case None => ""
+             }
           else ""
         }
 

docs/docs/reference/other-new-features/export.md

@@ -59,6 +59,8 @@ of one of the following forms:
 A member is _eligible_ if all of the following holds:
 
  - its owner is not a base class of the class(*) containing the export clause,
+ - the member does not override a concrete definition that has as owner
+   a base class of the class containing the export clause.
  - it is accessible at the export clause,
  - it is not a constructor, nor the (synthetic) class part of an object,
  - it is a given instance (or an old-style `implicit` value)

tests/neg/i7086.scala

@@ -0,0 +1,23 @@
+object Test1 {
+  class A {
+    override def toString: String = "A"
+  }
+  class B(a: A) {
+    export a.toString  // error: no eligible member toString at B.this.a
+  }
+}
+
+object Test extends App {
+  trait T {
+    def foo: Int = 1
+    def bar: Int
+  }
+  class A extends T {
+    override def foo = 2
+    override def bar = 2
+  }
+  class B(a: A) extends T {
+    export a.foo // error: no eligible member foo at B.this.a
+    export a.bar // OK
+  }
+}

tests/run/i7086.scala

@@ -0,0 +1,25 @@
+object Test1 {
+  class A {
+    override def toString: String = "A"
+  }
+  class B(a: A) {
+    export a._  // OK
+  }
+}
+
+object Test extends App {
+  trait T {
+    def foo: Int = 1
+    def bar: Int
+  }
+  class A extends T {
+    override def foo = 2
+    override def bar = 2
+  }
+  class B(a: A) extends T {
+    export a._
+  }
+  val b = B(A())
+  assert(b.foo == 1)
+  assert(b.bar == 2)
+}