Partial revert of scala#13780

scala#13780 caused several regressions and I think it is too restrictive as a fix.
I am reverting it an re-opening the original scala#11982 issue.

It would be good to get to the bottom of what the soundness problem hinted at
in scala#11982 is, and what a fix should be. As it stands scala#11982 is not obviously
a soundness problem but a separate compilation problem.

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

@@ -756,13 +756,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     }
 
     def tryBaseType(cls2: Symbol) = {
-      val allowBaseType = !caseLambda.exists || (tp1 match {
-        case tp: TypeRef if tp.symbol.isClass => true
-        case AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => true
-        case _ => false
-      })
       val base = nonExprBaseType(tp1, cls2)
-      if (base.exists && base.ne(tp1) && allowBaseType)
+      if (base.exists && (base `ne` tp1))
         isSubType(base, tp2, if (tp1.isRef(cls2)) approx else approx.addLow) ||
         base.isInstanceOf[OrType] && fourthTry
           // if base is a disjunction, this might have come from a tp1 type that
@@ -781,7 +776,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 || narrowGADTBounds(tp1, tp2, approx, isUpper = true))
               && (tp2.isAny || GADTusage(tp1.symbol))
 
-            !caseLambda.exists && isSubType(hi1, tp2, approx.addLow) || compareGADT || tryLiftedToThis1
+            isSubType(hi1, tp2, approx.addLow) || compareGADT || tryLiftedToThis1
           case _ =>
             // `Mode.RelaxedOverriding` is only enabled when checking Java overriding
             // in explicit nulls, and `Null` becomes a bottom type, which allows

compiler/test/dotc/pos-test-pickling.blacklist

@@ -42,6 +42,7 @@ i9999.scala
 13491.scala
 7512.scala
 i6505.scala
+i15155.scala
 
 # Opaque type
 i5720.scala

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -200,7 +200,6 @@ class CompilationTests {
   @Test def runAll: Unit = {
     implicit val testGroup: TestGroup = TestGroup("runAll")
     aggregateTests(
-      compileFile("tests/run-custom-args/typeclass-derivation1.scala", defaultOptions.without(yCheckOptions: _*)),
       compileFile("tests/run-custom-args/tuple-cons.scala", allowDeepSubtypes),
       compileFile("tests/run-custom-args/i5256.scala", allowDeepSubtypes),
       compileFile("tests/run-custom-args/no-useless-forwarders.scala", defaultOptions and "-Xmixin-force-forwarders:false"),

tests/neg/11982.scala

@@ -4,8 +4,8 @@ type Head[X] = X match {
 }
 
 object Unpair {
-  def unpair[X <: Tuple2[Any, Any]]: Head[X] = 1 // error
-  unpair[Tuple2["msg", 42]]: "msg"
+  def unpair[X <: Tuple2[Any, Any]]: Head[X] = 1
+  unpair[Tuple2["msg", 42]]: "msg"  // error
 }
 
 
@@ -14,8 +14,8 @@ type Head2[X] = X match {
 }
 
 object Unpair2 {
-  def unpair[X <: Tuple2[Tuple2[Any, Any], Tuple2[Any, Any]]]: Head2[X] = 1 // error
-  unpair[Tuple2[Tuple2["msg", 42], Tuple2[41, 40]]]: "msg"
+  def unpair[X <: Tuple2[Tuple2[Any, Any], Tuple2[Any, Any]]]: Head2[X] = 1
+  unpair[Tuple2[Tuple2["msg", 42], Tuple2[41, 40]]]: "msg" // error
 }
 
 
@@ -35,6 +35,6 @@ type Head4[X] = X match {
 }
 
 object Unpair4 {
-  def unpair[X <: Foo[Any, Any]]: Head4[Foo[X, X]] = 1 // error
-  unpair[Foo["msg", 42]]: "msg"
+  def unpair[X <: Foo[Any, Any]]: Head4[Foo[X, X]] = 1
+  unpair[Foo["msg", 42]]: "msg" // error
 }

tests/neg/6570-1.scala

@@ -33,21 +33,7 @@ class Asploder extends Root[Cov[Box[Int & String]]] {
 }
 
 object Main {
-  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing // error
-  //                                                ^^^^^^^
-  //       Found:    M[T]
-  //       Required: Trait2
-  //
-  //       where:    T is a type in method foo with bounds <: Cov[Box[Int]]
-  //
-  //
-  //       Note: a match type could not be fully reduced:
-  //
-  //         trying to reduce  M[T]
-  //         failed since selector  T
-  //         does not match  case Cov[x] => N[x]
-  //         and cannot be shown to be disjoint from it either.
-
+  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing
   def explode = foo(new Asploder)
 
   def main(args: Array[String]): Unit =

tests/neg/6570.scala

@@ -21,7 +21,7 @@ object UpperBoundParametricVariant {
   trait Child[A <: Cov[Int]] extends Root[A]
 
   // we reduce `M[T]` to `Trait2`, even though we cannot be certain of that
-  def foo[T <: Cov[Int]](c: Child[T]): Trait2 = c.thing // error
+  def foo[T <: Cov[Int]](c: Child[T]): Trait2 = c.thing
 
   class Asploder extends Child[Cov[String & Int]] {
     def thing = new Trait1 {} // error
@@ -42,7 +42,7 @@ object InheritanceVariant {
 
   trait Child extends Root { type B <: { type A <: Int } }
 
-  def foo(c: Child): Trait2 = c.thing // error
+  def foo(c: Child): Trait2 = c.thing
 
   class Asploder extends Child {
     type B = { type A = String & Int }
@@ -98,7 +98,7 @@ object UpperBoundVariant {
 
   trait Child extends Root { type A <: Cov[Int] }
 
-  def foo(c: Child): Trait2 = c.thing // error
+  def foo(c: Child): Trait2 = c.thing
 
   class Asploder extends Child {
     type A = Cov[String & Int]

tests/neg/i15155.scala → tests/pos/i15155.scala

@@ -7,5 +7,5 @@ type EnumValue[A <: Enumeration] = A match {
 
 // https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala/org/json4s/ext/EnumSerializer.scala#L25-L26
 class EnumSerializer[E <: Enumeration: ClassTag](enumeration: E) {
-  val EnumerationClass = classOf[EnumValue[E]] // error
+  val EnumerationClass = classOf[EnumValue[E]]
 }

tests/run-custom-args/typeclass-derivation1.scala → tests/run/typeclass-derivation1.scala

@@ -96,15 +96,3 @@ object Test extends App {
   assert(!eq2.equals(yss, xss))
   assert(eq2.equals(yss, yss))
 }
-
-// -Ycheck failure minimized to:
-// import scala.compiletime.*
-// object Eq {
-//   inline def deriveForProduct[Elems <: Tuple](xs: Elems): Boolean = inline erasedValue[Elems] match {
-//     case _: (elem *: elems1) =>
-//       val xs1 = xs.asInstanceOf[elem *: elems1]
-//       deriveForProduct(xs1.tail)
-//     case _: EmptyTuple =>
-//       true
-//   }
-// }