Don't follow BaseType of abstract binders in MT reduction

Fix #11982 and the associated soundness problem. The issue with the
behavior on master arises from the fact that type binder of match types
might change as context gets more precise, which results in a single
match type reducing in two different ways. This issue comes from the
fact that subtyping looks into base types, and is thus able to match a
type such as `T <: Tuple2[Int, Int]` against a pattern `case Tuple2[a,
b]`, even if the best solutions for `a` and `b` in the current context
are not guaranteed to be the best solution in more precise contexts
(such as at call site in the added test case).

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

@@ -756,8 +756,13 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     }
 
     def tryBaseType(cls2: Symbol) = {
+      val allowBaseType = caseLambda.eq(NoType) || (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))
+      if (base.exists && base.ne(tp1) && allowBaseType)
         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
@@ -776,7 +781,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 || narrowGADTBounds(tp1, tp2, approx, isUpper = true))
               && (tp2.isAny || GADTusage(tp1.symbol))
 
-            isSubType(hi1, tp2, approx.addLow) || compareGADT || tryLiftedToThis1
+            caseLambda.eq(NoType) && isSubType(hi1, tp2, approx.addLow) || compareGADT || tryLiftedToThis1
           case _ =>
             def isNullable(tp: Type): Boolean = tp.widenDealias match {
               case tp: TypeRef => tp.symbol.isNullableClass
@@ -2536,7 +2541,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       override def apply(x: Boolean, t: Type) =
         x && {
           t match {
-            case tp: TypeRef if tp.symbol.isAbstractOrParamType => false
+            case tp: TypeRef if !tp.symbol.isClass => false
             case _: SkolemType | _: TypeVar | _: TypeParamRef => false
             case _ => foldOver(x, t)
           }

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

@@ -1124,7 +1124,7 @@ trait Implicits:
     /** The expected type where parameters and uninstantiated typevars are replaced by wildcard types */
     val wildProto: Type =
       if argument.isEmpty then wildApprox(pt)
-      else ViewProto(wildApprox(argument.tpe.widen), wildApprox(pt))
+      else ViewProto(wildApprox(argument.tpe.widen.normalized), wildApprox(pt))
         // Not clear whether we need to drop the `.widen` here. All tests pass with it in place, though.
 
     val isNotGiven: Boolean = wildProto.classSymbol == defn.NotGivenClass

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

@@ -193,6 +193,7 @@ 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/fors.scala", defaultOptions.and("-source", "future")),

tests/neg/11982.scala

@@ -0,0 +1,40 @@
+// testCompilation 11982.scala
+type Head[X] = X match {
+  case Tuple2[a, b] => a
+}
+
+object Unpair {
+  def unpair[X <: Tuple2[Any, Any]]: Head[X] = 1 // error
+  unpair[Tuple2["msg", 42]]: "msg"
+}
+
+
+type Head2[X] = X match {
+  case Tuple2[Tuple2[a, b], Tuple2[c, d]] => a
+}
+
+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"
+}
+
+
+type Head3[X] = X match {
+  case Tuple2[a, b] => a
+}
+
+object Unpair3 {
+  def unpair[X <: Tuple2[Any, Any]]: Head3[Tuple2[X, X]] = (1, 2) // error
+  unpair[Tuple2["msg", 42]]: ("msg", 42)
+}
+
+trait Foo[+A, +B]
+
+type Head4[X] = X match {
+  case Foo[Foo[a, b], Foo[c, d]] => a
+}
+
+object Unpair4 {
+  def unpair[X <: Foo[Any, Any]]: Head4[Foo[X, X]] = 1 // error
+  unpair[Foo["msg", 42]]: "msg"
+}

tests/neg/6570-1.scala

@@ -21,10 +21,32 @@ trait Root[A] {
 
 class Asploder extends Root[Cov[Box[Int & String]]] {
   def thing = new Trait1 {} // error
+  //                       ^
+  //                   Found:    Object with Trait1 {...}
+  //                   Required: N[Box[Int & String]]
+  //
+  //                   Note: a match type could not be fully reduced:
+  //
+  //                     trying to reduce  N[Box[Int & String]]
+  //                     failed since selector  Box[Int & String]
+  //                     is uninhabited (there are no values of that type).
 }
 
 object Main {
-  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing
+  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 explode = foo(new Asploder)
 

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
+  def foo[T <: Cov[Int]](c: Child[T]): Trait2 = c.thing // error
 
   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
+  def foo(c: Child): Trait2 = c.thing // error
 
   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
+  def foo(c: Child): Trait2 = c.thing // error
 
   class Asploder extends Child {
     type A = Cov[String & Int]

tests/pos/11982-a/119_1.scala

@@ -0,0 +1,9 @@
+object Unpair {
+  class Inv[T]
+
+  type Head[X] = X match {
+    case Tuple2[a, b] => a
+  }
+
+  def unpair[X <: Tuple2[Any, Any]]: Head[X] = ???
+}

tests/pos/11982-a/119_2.scala

@@ -0,0 +1,5 @@
+object UnpairApp {
+  import Unpair._
+
+  val x: String = unpair[("msg", 42)]
+}

tests/pos/13491.scala

@@ -86,7 +86,8 @@ object Rule {
   type RuleN[+L <: HList]   = Rule[HNil, L]
 
   def rule[I <: HList, O <: HList](r: Rule[I, O]): Rule[I, O] = ???
-  implicit def valueMap[T](m: Map[String, T])(implicit h: HListable[T]): RuleN[h.Out] = ???
+
+  implicit def valueMap[T, Out0 <: HList](m: Map[String, T])(implicit h: HListable[T] { type Out = Out0 }): RuleN[Out0] = ???
 }
 
 object Test {

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

@@ -96,3 +96,15 @@ 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
+//   }
+// }