Merge pull request #11077 from dotty-staging/fix-#11072

Better error message if derived trait does not have companion

Author: odersky

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) {

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

tests/neg/i11072.scala

@@ -0,0 +1,5 @@
+trait Sealed[A]
+
+enum Foo derives Sealed { // error
+  case A, B, C
+}