Fix check whether classtag can be generated for match types

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

@@ -520,8 +520,9 @@ object TypeErasure {
         case _: ClassInfo => true
         case _ => false
       }
-    case tp: TypeParamRef => false
-    case tp: TypeBounds => false
+    case _: TypeParamRef => false
+    case _: TypeBounds => false
+    case _: MatchType => false
     case tp: TypeProxy => hasStableErasure(tp.translucentSuperType)
     case tp: AndType => hasStableErasure(tp.tp1) && hasStableErasure(tp.tp2)
     case tp: OrType  => hasStableErasure(tp.tp1) && hasStableErasure(tp.tp2)

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

@@ -1071,7 +1071,7 @@ object Types {
      *  @param relaxedCheck   if true type `Null` becomes a subtype of non-primitive value types in TypeComparer.
      *  @param matchLoosely   if true the types `=> T` and `()T` are seen as overriding each other.
      *  @param checkClassInfo if true we check that ClassInfos are within bounds of abstract types
-     * 
+     *
      *  @param isSubType      a function used for checking subtype relationships.
      */
     final def overrides(that: Type, relaxedCheck: Boolean, matchLoosely: => Boolean, checkClassInfo: Boolean = true,

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

@@ -52,14 +52,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
             if defn.SpecialClassTagClasses.contains(sym) then
               classTagModul.select(sym.name.toTermName).withSpan(span)
             else
-              def clsOfType(tp: Type): Type = tp.dealias.underlyingMatchType match
-                case matchTp: MatchType =>
-                  matchTp.alternatives.map(clsOfType) match
-                    case ct1 :: cts if cts.forall(ct1 == _) => ct1
-                    case _ => NoType
-                case _ =>
-                  escapeJavaArray(erasure(tp))
-              val ctype = clsOfType(tp)
+              val ctype = escapeJavaArray(erasure(tp))
               if ctype.exists then
                 classTagModul.select(nme.apply)
                   .appliedToType(tp)

tests/neg/i15618.check

@@ -16,3 +16,9 @@
    |
    |                                                case Float32 => Float
    |                                                case Int32 => Int
+-- [E172] Type Error: tests/neg/i15618.scala:21:33 ---------------------------------------------------------------------
+21 |  def toArray: Array[T] = Array() // error
+   |                                 ^
+   |                                 No ClassTag available for T
+   |
+   |                                 where:    T is a type in class Tensor2 with bounds <: Int | Float

tests/neg/i15618.scala

@@ -16,8 +16,16 @@ class Tensor[T <: DType](dtype: T):
   def toSeq: Seq[ScalaType[T]] = Seq()
   def toArray: Array[ScalaType[T]] = Array() // error
 
+class Tensor2[T <: Int | Float](dtype: T):
+  def toSeq: Seq[T] = Seq()
+  def toArray: Array[T] = Array() // error
+
 @main
 def Test =
   val t = Tensor(Float32) // Tensor[Float32]
   println(t.toSeq.headOption) // works, Seq[Float]
   println(t.toArray.headOption) // ClassCastException
+
+  val t2 = Tensor2(0.0f) // Tensor2[Float]
+  println(t.toSeq.headOption)
+  println(t.toArray.headOption)

tests/pos/i16706.scala

@@ -0,0 +1,17 @@
+import scala.deriving.Mirror
+import scala.reflect.ClassTag
+
+type TupleUnionLub[T <: Tuple, Lub, Acc <: Lub] <: Lub = T match {
+  case (h & Lub) *: t => TupleUnionLub[t, Lub, Acc | h]
+  case EmptyTuple     => Acc
+}
+
+transparent inline given derived[A](
+    using m: Mirror.SumOf[A],
+    idClassTag: ClassTag[TupleUnionLub[m.MirroredElemTypes, A, Nothing]]
+): Unit = ()
+
+sealed trait Foo
+case class FooA(a: Int) extends Foo
+
+val instance = derived[Foo] // error

tests/pos/i16707.scala

@@ -0,0 +1,11 @@
+import scala.deriving.Mirror
+import scala.reflect.ClassTag
+
+transparent inline given derived[A](
+      using m: Mirror.ProductOf[A],
+      idClassTag: ClassTag[Tuple.Union[m.MirroredElemTypes]]
+  ): Unit = ???
+
+case class Foo(a: Int)
+
+val instance = derived[Foo] // error