Backport "Add special handling for comparisons with Singleton types" to LTS (#21126)

Backports #20474 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

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

@@ -920,12 +920,15 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         compareAppliedType1(tp1, tycon1, args1)
       case tp1: SingletonType =>
         def comparePaths = tp2 match
-          case tp2: TermRef =>
+          case tp2: (TermRef | ThisType) =>
             compareAtoms(tp1, tp2, knownSingletons = true).getOrElse(false)
-            || { // needed to make from-tasty work. test cases: pos/i1753.scala, pos/t839.scala
-              tp2.info.widenExpr.dealias match
-                case tp2i: SingletonType => recur(tp1, tp2i)
-                case _ => false
+            || {
+              // If tp2's underlying type tp2super is also effectively a singleton, compare
+              // against that. The idea is that if tp1 <: tp2super and tp2 <: tp2super and
+              // tp2super is also singleton, then tp1 and tp2 must be the same singleton.
+              // Needed to make from-tasty work. test cases: pos/i1753.scala, pos/t839.scala
+              val tp2super = tp2.superType.widenExpr
+              tp2super.isEffectivelySingleton && recur(tp1, tp2super)
             }
           case _ => false
 

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

@@ -326,6 +326,16 @@ object Types extends TypeUtils {
     /** Is this type a (possibly aliased) singleton type? */
     def isSingleton(using Context): Boolean = dealias.isInstanceOf[SingletonType]
 
+    /** Is this type a (possibly aliased) singleton type or a type proxy
+     *  or Or/And type known to be a singleton type?
+     */
+    def isEffectivelySingleton(using Context): Boolean = dealias match
+      case tp: SingletonType => true
+      case tp: TypeProxy => tp.superType.isEffectivelySingleton
+      case AndType(tpL, tpR) => tpL.isEffectivelySingleton || tpR.isEffectivelySingleton
+      case OrType(tpL, tpR) => tpL.isEffectivelySingleton && tpR.isEffectivelySingleton
+      case _ => false
+
     /** Is this type of kind `AnyKind`? */
     def hasAnyKind(using Context): Boolean = {
       @tailrec def loop(tp: Type): Boolean = tp match {

tests/pos/i15030.scala

@@ -0,0 +1,16 @@
+sealed trait Schema[A]
+
+object Schema extends RecordInstances:
+  case class Field[A]()
+
+sealed trait RecordInstances:
+  self: Schema.type =>
+
+  case class Record[A](field: Field[A]) extends Schema[A]
+
+import Schema._
+
+val field: Field[Int] = Field()
+
+// Uh oh Found Playground.Schema.Field[Int] but Requried RecordInstances.this.Field[Int]
+val record = Record[Int](field)