Elide companion defs to a object extending AnyVal

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -820,7 +820,7 @@ object desugar {
       }
       else if (companionMembers.nonEmpty || companionDerived.nonEmpty || isEnum)
         companionDefs(anyRef, companionMembers)
-      else if (isValueClass)
+      else if isValueClass && !isObject then
         companionDefs(anyRef, Nil)
       else Nil
 

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

@@ -738,7 +738,10 @@ object Checking {
               for (p <- params if !p.is(Erased))
                 report.error("value class can only have one non `erased` parameter", p.srcPos)
           case Nil =>
-            report.error(ValueClassNeedsOneValParam(clazz), clazz.srcPos)
+            if clazz.is(Module) then
+              report.error("A module cannot extends AnyVal",clazz.srcPos)
+            else
+              report.error(ValueClassNeedsOneValParam(clazz), clazz.srcPos)
         }
       }
       stats.foreach(checkValueClassMember)

tests/neg/i18274.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i18274.scala:1:7 -----------------------------------------------------------------------------------
+1 |object Foo extends AnyVal // error
+  |       ^
+  |       A module cannot extends AnyVal

tests/neg/i18274.scala

@@ -0,0 +1 @@
+object Foo extends AnyVal // error