alternative design with a class - cleaner bytecode and less duplication

Author: bishabosha

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

@@ -1597,9 +1597,8 @@ object desugar {
         AppliedTypeTree(ref(defn.NamedTupleTypeRef), namesTuple :: tup :: Nil)
       else
         val Builder_apply = Select(Select(ref(defn.NamedTupleModule), nme.NamedTupleBuilder), nme.apply)
-        val build = Select(ref(defn.NamedTupleModule), nme.build)
         val builder = TypeApply(Builder_apply, namesTuple :: Nil)
-        Apply(Apply(build, builder :: Nil), tup :: Nil)
+        Apply(Select(builder, nme.build), tup :: Nil)
 
   /** When desugaring a list pattern arguments `elems` adapt them and the
    *  expected type `pt` to each other. This means:

library/src/scala/NamedTuple.scala

@@ -19,13 +19,8 @@ object NamedTuple:
 
   def unapply[N <: Tuple, V <: Tuple](x: NamedTuple[N, V]): Some[V] = Some(x)
 
-  object NamedTupleBuilder:
-    opaque type Builder[N <: Tuple] = Unit
-
-    @annotation.compileTimeOnly("NamedTupleBuilder.apply should be used as the argument to NamedTuple.build")
-    def apply[N <: Tuple]: Builder[N] = ???
-
-  extension [N <: Tuple](inline builder: NamedTupleBuilder.Builder[N])
+  @annotation.compileTimeOnly("NamedTupleBuilder.Builder should have been eliminated by calling the build method.")
+  final class NamedTupleBuilder[N <: Tuple]():
     inline def build[V <: Tuple](inline x: V): NamedTuple[N, V] = x
 
   extension [V <: Tuple](x: V)