Fix #2470: Do not desugar companion for AnyVal object

Author: nicolasstucki

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

@@ -461,8 +461,12 @@ object desugar {
       }
       else if (defaultGetters.nonEmpty)
         companionDefs(anyRef, defaultGetters)
-      else if (isValueClass)
-        companionDefs(anyRef, Nil)
+      else if (isValueClass) {
+        constr0.vparamss match {
+          case List(_ :: Nil) => companionDefs(anyRef, Nil)
+          case _ => Nil // error will be emitted in typer
+        }
+      }
       else Nil
 
     // For an implicit class C[Ts](p11: T11, ..., p1N: T1N) ... (pM1: TM1, .., pMN: TMN), the method

tests/neg/i2470.scala

@@ -0,0 +1 @@
+object Foo extends AnyVal // error: value class needs to have exactly one val parameter