Heal member-select on opaque reference

When the prefix of an opaque isn't the .this reference of the module
class, then its RHS isn't visible.  TypeComparer uses ctx.owner to
"heal" or "lift" this type such that it is.  We reuse that logic for
member selection.

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

@@ -712,6 +712,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         typedSelectWithAdapt(tree, pt, qual)
       else EmptyTree
 
+    def tryLiftToThis() =
+      val wtp = qual.tpe.widen
+      val liftedTp = comparing(_.liftToThis(wtp))
+      if liftedTp ne wtp then
+        val qual1 = qual.cast(liftedTp)
+        val tree1 = cpy.Select(tree0)(qual1, selName)
+        val rawType1 = selectionType(tree1, qual1)
+        tryType(tree1, qual1, rawType1)
+      else EmptyTree
+
     def trySmallGenericTuple(tree: untpd.Select, qual: Tree, withCast: Boolean) =
       if qual.tpe.isSmallGenericTuple then
         if withCast then
@@ -761,6 +771,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     tryType(tree, qual, rawType)
       .orElse(trySimplifyApply())
       .orElse(tryInstantiateTypeVar())
+      .orElse(tryLiftToThis())
       .orElse(trySmallGenericTuple(tree, qual, withCast = true))
       .orElse(tryExt(tree, qual))
       .orElse(tryGadt(tree))

tests/pos/i19609.orig.scala

@@ -0,0 +1,12 @@
+object o {
+  opaque type T = String
+
+  summon[o.T =:= T]      // OK
+  summon[o.T =:= String] // OK
+
+  def test1(t: T): Int =
+    t.length // OK
+
+  def test2(t: o.T): Int =
+    t.length // Error: value length is not a member of Playground.o.T
+}

tests/pos/i19609.scala

@@ -0,0 +1,24 @@
+object o { u =>
+  opaque type T = String
+
+  def st = summon[String =:= T]
+  def su = summon[String =:= u.T]
+  def so = summon[String =:= o.T]
+
+  def ts = summon[T =:= String]
+  def tu = summon[T =:= u.T]
+  def to = summon[T =:= o.T]
+
+  def us = summon[u.T =:= String]
+  def ut = summon[u.T =:= T]
+  def uo = summon[u.T =:= o.T]
+
+  def os = summon[o.T =:= String]
+  def ot = summon[o.T =:= T]
+  def ou = summon[o.T =:= u.T]
+
+  def ms(x: String): Int = x.length // ok
+  def mt(x:      T): Int = x.length // ok
+  def mu(x:    u.T): Int = x.length // ok
+  def mo(x:    o.T): Int = x.length // was: error: value length is not a member of o.T
+}