Fix implicit ValueOf synthesis

Foxes #12583

Author: odersky

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

@@ -163,7 +163,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
     formal.argInfos match
       case arg :: Nil =>
-        fullyDefinedType(arg.dealias, "ValueOf argument", span).normalized match
+        fullyDefinedType(arg, "ValueOf argument", span).normalized.dealias match
           case ConstantType(c: Constant) =>
             success(Literal(c))
           case TypeRef(_, sym) if sym == defn.UnitClass =>

tests/pos/i12583.scala

@@ -0,0 +1,5 @@
+case class Wrapper[A <: Singleton](a: A)(using ValueOf[A])
+
+def test[A <: Singleton](blabla: Wrapper[A]): Unit = ()
+
+val _ = test(Wrapper("yo"))