From 32d981e7c033dee82af24cc19b51e41f6378f4f2 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Wed, 19 Feb 2020 15:37:55 +0100
Subject: [PATCH] Improve overriding error message for exports

---
 .../dotty/tools/dotc/typer/RefChecks.scala    |  2 ++
 tests/neg/exports1.scala                      | 20 +++++++++++++++++++
 tests/neg/exports2.check                      |  5 +++++
 tests/neg/exports2.scala                      |  9 +++++++++
 4 files changed, 36 insertions(+)
 create mode 100644 tests/neg/exports1.scala
 create mode 100644 tests/neg/exports2.check
 create mode 100644 tests/neg/exports2.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index a5315400efb1..cecd78f838fc 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/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))
diff --git a/tests/neg/exports1.scala b/tests/neg/exports1.scala
new file mode 100644
index 000000000000..ecd89c694043
--- /dev/null
+++ b/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
diff --git a/tests/neg/exports2.check b/tests/neg/exports2.check
new file mode 100644
index 000000000000..6b35b1fe4621
--- /dev/null
+++ b/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
diff --git a/tests/neg/exports2.scala b/tests/neg/exports2.scala
new file mode 100644
index 000000000000..913248a2041a
--- /dev/null
+++ b/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
+