Optimise refineUsingParent to use constrained

Using `constrained` on a TypeLambda means adding one TypeLambda for all
the type parameters in tp1, while newTypeVar creates a TypeLambda for
each type parameter.

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -902,7 +902,9 @@ object TypeOps:
     }
 
     val inferThisMap = new InferPrefixMap
-    val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds, DepParamName.fresh(tparam.paramName)) }
+    val tvars = tp1.etaExpand match
+      case eta: TypeLambda => constrained(eta)
+      case _               => Nil
     val protoTp1 = inferThisMap.apply(tp1).appliedTo(tvars)
 
     if gadtSyms.nonEmpty then

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4395,9 +4395,11 @@ object Types extends TypeUtils {
 
     /** Distributes Lambda inside type bounds. Examples:
      *
-     *      type T[X] = U        becomes    type T = [X] -> U
-     *      type T[X] <: U       becomes    type T >: Nothing <: ([X] -> U)
-     *      type T[X] >: L <: U  becomes    type T >: ([X] -> L) <: ([X] -> U)
+     *  {{{
+     *      type T[X] = U        becomes    type T = [X] =>> U
+     *      type T[X] <: U       becomes    type T >: Nothing <: ([X] =>> U)
+     *      type T[X] >: L <: U  becomes    type T >: ([X] =>> L) <: ([X] =>> U)
+     *  }}}
      *
      *  The variances of regular TypeBounds types, as well as of match aliases
      *  and of opaque aliases are always determined from the given parameters
@@ -4409,13 +4411,15 @@ object Types extends TypeUtils {
      *
      *  Examples:
      *
+     *  {{{
      *    type T[X] >: A              // X is invariant
      *    type T[X] <: List[X]        // X is invariant
      *    type T[X] = List[X]         // X is covariant (determined structurally)
      *    opaque type T[X] = List[X]  // X is invariant
      *    opaque type T[+X] = List[X] // X is covariant
      *    type T[A, B] = A => B       // A is contravariant, B is covariant (determined structurally)
      *    type T[A, +B] = A => B      // A is invariant, B is covariant
+     *  }}}
      */
     def boundsFromParams[PI <: ParamInfo.Of[TypeName]](params: List[PI], bounds: TypeBounds)(using Context): TypeBounds = {
       def expand(tp: Type, useVariances: Boolean) =