Backport #15625: Fix two problems related to match types as array elements

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

@@ -328,6 +328,9 @@ object TypeErasure {
         isGenericArrayElement(tp.alias, isScala2)
       case tp: TypeBounds =>
         !fitsInJVMArray(tp.hi)
+      case tp: MatchType =>
+        val alts = tp.alternatives
+        alts.nonEmpty && !fitsInJVMArray(alts.reduce(OrType(_, _, soft = true)))
       case tp: TypeProxy =>
         isGenericArrayElement(tp.translucentSuperType, isScala2)
       case tp: AndType =>

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

@@ -428,11 +428,13 @@ object Types {
 
     /** Is this a match type or a higher-kinded abstraction of one?
      */
-    def isMatch(using Context): Boolean = stripped match {
-      case _: MatchType => true
-      case tp: HKTypeLambda => tp.resType.isMatch
-      case tp: AppliedType => tp.isMatchAlias
-      case _ => false
+    def isMatch(using Context): Boolean = underlyingMatchType.exists
+
+    def underlyingMatchType(using Context): Type = stripped match {
+      case tp: MatchType => tp
+      case tp: HKTypeLambda => tp.resType.underlyingMatchType
+      case tp: AppliedType if tp.isMatchAlias => tp.superType.underlyingMatchType
+      case _ => NoType
     }
 
     /** Is this a higher-kinded type lambda with given parameter variances? */

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

@@ -28,26 +28,35 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   private type SpecialHandlers = List[(ClassSymbol, SpecialHandler)]
 
   val synthesizedClassTag: SpecialHandler = (formal, span) =>
-    formal.argInfos match
-      case arg :: Nil =>
-        if isFullyDefined(arg, ForceDegree.all) then
-          arg match
-            case defn.ArrayOf(elemTp) =>
-              val etag = typer.inferImplicitArg(defn.ClassTagClass.typeRef.appliedTo(elemTp), span)
-              if etag.tpe.isError then EmptyTreeNoError else withNoErrors(etag.select(nme.wrap))
-            case tp if hasStableErasure(tp) && !defn.isBottomClassAfterErasure(tp.typeSymbol) =>
-              val sym = tp.typeSymbol
-              val classTag = ref(defn.ClassTagModule)
-              val tag =
-                if defn.SpecialClassTagClasses.contains(sym) then
-                  classTag.select(sym.name.toTermName)
-                else
-                  val clsOfType = escapeJavaArray(erasure(tp))
-                  classTag.select(nme.apply).appliedToType(tp).appliedTo(clsOf(clsOfType))
-              withNoErrors(tag.withSpan(span))
-            case tp => EmptyTreeNoError
-        else EmptyTreeNoError
-      case _ => EmptyTreeNoError
+    val tag = formal.argInfos match
+      case arg :: Nil if isFullyDefined(arg, ForceDegree.all) =>
+        arg match
+          case defn.ArrayOf(elemTp) =>
+            val etag = typer.inferImplicitArg(defn.ClassTagClass.typeRef.appliedTo(elemTp), span)
+            if etag.tpe.isError then EmptyTree else etag.select(nme.wrap)
+          case tp if hasStableErasure(tp) && !defn.isBottomClassAfterErasure(tp.typeSymbol) =>
+            val sym = tp.typeSymbol
+            val classTagModul = ref(defn.ClassTagModule)
+            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)
+              if ctype.exists then
+                classTagModul.select(nme.apply)
+                  .appliedToType(tp)
+                  .appliedTo(clsOf(ctype))
+                  .withSpan(span)
+              else EmptyTree
+          case _ => EmptyTree
+      case _ => EmptyTree
+    (tag, Nil)
   end synthesizedClassTag
 
   val synthesizedTypeTest: SpecialHandler =

tests/neg/i15618.check

@@ -0,0 +1,18 @@
+-- Error: tests/neg/i15618.scala:17:44 ---------------------------------------------------------------------------------
+17 |  def toArray: Array[ScalaType[T]] = Array() // error
+   |                                            ^
+   |                                            No ClassTag available for ScalaType[T]
+   |
+   |                                            where:    T is a type in class Tensor with bounds <: DType
+   |
+   |
+   |                                            Note: a match type could not be fully reduced:
+   |
+   |                                              trying to reduce  ScalaType[T]
+   |                                              failed since selector  T
+   |                                              does not match  case Float16 => Float
+   |                                              and cannot be shown to be disjoint from it either.
+   |                                              Therefore, reduction cannot advance to the remaining cases
+   |
+   |                                                case Float32 => Float
+   |                                                case Int32 => Int

tests/neg/i15618.scala

@@ -0,0 +1,23 @@
+sealed abstract class DType
+sealed class Float16 extends DType
+sealed class Float32 extends DType
+sealed class Int32 extends DType
+
+object Float16 extends Float16
+object Float32 extends Float32
+object Int32 extends Int32
+
+type ScalaType[U <: DType] <: Int | Float = U match
+  case Float16 => Float
+  case Float32 => Float
+  case Int32 => Int
+
+class Tensor[T <: DType](dtype: T):
+  def toSeq: Seq[ScalaType[T]] = Seq()
+  def toArray: Array[ScalaType[T]] = Array() // error
+
+@main
+def Test =
+  val t = Tensor(Float32) // Tensor[Float32]
+  println(t.toSeq.headOption) // works, Seq[Float]
+  println(t.toArray.headOption) // ClassCastException

tests/run/i15618.check

@@ -0,0 +1 @@
+Some(1)

tests/run/i15618.scala

@@ -0,0 +1,24 @@
+sealed abstract class DType
+sealed class Float16 extends DType
+sealed class Float32 extends DType
+sealed class Int32 extends DType
+
+object Float16 extends Float16
+object Float32 extends Float32
+object Int32 extends Int32
+
+type ScalaType[U <: DType] <: Int | Float = U match
+  case Float16 => Float
+  case Float32 => Float
+  case Int32 => Int
+
+abstract class Tensor[T <: DType]:
+  def toArray: Array[ScalaType[T]]
+
+object IntTensor extends Tensor[Int32]:
+  def toArray: Array[Int] = Array(1, 2, 3)
+
+@main
+def Test =
+  val t = IntTensor: Tensor[Int32]
+  println(t.toArray.headOption) // was ClassCastException