From 4e34f948fdcab8d2a67a2d9d5ac3ff1b75d944f9 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 12 Jan 2021 17:54:51 +0100
Subject: [PATCH] Better error message if derived trait does not have companion

Fixes #11072
---
 compiler/src/dotty/tools/dotc/typer/Deriving.scala | 6 ++++--
 tests/neg/i11072.check                             | 4 ++++
 tests/neg/i11072.scala                             | 5 +++++
 3 files changed, 13 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i11072.check
 create mode 100644 tests/neg/i11072.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Deriving.scala b/compiler/src/dotty/tools/dotc/typer/Deriving.scala
index 1218d2c10094..269924c21114 100644
--- a/compiler/src/dotty/tools/dotc/typer/Deriving.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Deriving.scala
@@ -291,9 +291,11 @@ trait Deriving {
               companionRef(tp.underlying)
           }
           val resultType = instantiated(sym.info)
-          val module = untpd.ref(companionRef(resultType)).withSpan(sym.span)
+          val companion = companionRef(resultType)
+          val module = untpd.ref(companion).withSpan(sym.span)
           val rhs = untpd.Select(module, nme.derived)
-          typed(rhs, resultType)
+          if companion.termSymbol.exists then typed(rhs, resultType)
+          else errorTree(rhs, em"$resultType cannot be derived since ${resultType.typeSymbol} has no companion object")
       end typeclassInstance
 
       def syntheticDef(sym: Symbol): Tree = inContext(ctx.fresh.setOwner(sym).setNewScope) {
diff --git a/tests/neg/i11072.check b/tests/neg/i11072.check
new file mode 100644
index 000000000000..7f1e1d503c67
--- /dev/null
+++ b/tests/neg/i11072.check
@@ -0,0 +1,4 @@
+-- Error: tests/neg/i11072.scala:3:17 ----------------------------------------------------------------------------------
+3 |enum Foo derives Sealed { // error
+  |                 ^
+  |                 Sealed[Foo] cannot be derived since trait Sealed has no companion object
diff --git a/tests/neg/i11072.scala b/tests/neg/i11072.scala
new file mode 100644
index 000000000000..5df4cfbfb3aa
--- /dev/null
+++ b/tests/neg/i11072.scala
@@ -0,0 +1,5 @@
+trait Sealed[A]
+
+enum Foo derives Sealed { // error
+  case A, B, C
+}
\ No newline at end of file