Avoid loosing denotations of named types during integrate

[mbovel]

No description provided.

[odersky]

Otherwise LGTM

[odersky]

This is duplicates code from SubstMap, but at lower performance. I think we should factor out only what is essential here:

• Make Substituters.SubstMap non-final and note that it is overridden by IntergrateMap in the comment.

• Define IntegrateMap as

  private class IntegrateMap(params: List[Symbol], paramRefs: 
    List[Type])(using Context) extends Substituters.SubstMap(params, paramrefs):
    override def derivedSelect(tp: NamedType, pre: Type): Type = ...

[mbovel]

As discussed orally, we can't extend SubstMap, because it uses subst which directly calls Type#derivedSelect , not TypeMap#derivedSelect:

scala3/compiler/src/dotty/tools/dotc/core/Substituters.scala

Line 69 in 9bef68c

else tp.derivedSelect(subst(tp.prefix, from, to, theMap))

I instead inlined the code from SubstMap as you suggested, done in 9bef68c.

[mbovel]

As a side-effect, that also restores the original behavior for the test i12448, which my previous implementation changed:

scala3/tests/neg/i12448.scala

Lines 1 to 5 in 7976598

	object Main {
	def mkArray[T <: A]: T#AType // error // error
	mkArray[Array] // was: "assertion failed: invalid prefix HKTypeLambda..." // error
	val x = mkArray[Array] // error
	}

This is the output with the SubstMap implementation:

sbt:scala3> scalac tests/neg/i12448.scala
...
-- [E006] Not Found Error: tests/neg/i12448.scala:2:19 -------------------------
2 |  def mkArray[T <: A]: T#AType // error // error
  |                   ^
  |                   Not found: type A
  |
  | longer explanation available when compiling with `-explain`
-- [E067] Syntax Error: tests/neg/i12448.scala:2:6 -----------------------------
2 |  def mkArray[T <: A]: T#AType // error // error
  |      ^
  |Declaration of method mkArray not allowed here: only classes can have declared but undefined members
-- Error: tests/neg/i12448.scala:3:9 -------------------------------------------
3 |  mkArray[Array] // was: "assertion failed: invalid prefix HKTypeLambda..." // error
  |         ^
  |         invalid new prefix Array cannot replace T in type T#AType
-- Error: tests/neg/i12448.scala:4:17 ------------------------------------------
4 |  val x = mkArray[Array] // error
  |                 ^
  |                 invalid new prefix Array cannot replace T in type T#AType
4 errors found

while with my previous implementation, the two invalid prefix errors did not appear.