Disallow some opaque type aliases in match type patterns

Disallow opaque type aliases in match type patterns in scopes where the alias is visible.

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

@@ -5211,12 +5211,14 @@ object Types extends TypeUtils {
 
   enum MatchTypeCaseError:
     case Alias(sym: Symbol)
+    case OpaqueAlias(sym: Symbol)
     case RefiningBounds(name: TypeName)
     case StructuralType(name: TypeName)
     case UnaccountedTypeParam(name: TypeName)
 
     def explanation(using Context) = this match
       case Alias(sym) => i"a type alias `${sym.name}`"
+      case OpaqueAlias(sym) => i"an opaque type alias `${sym.name}` in the scope where its alias is known"
       case RefiningBounds(name) => i"an abstract type member `$name` with bounds that need verification"
       case StructuralType(name) => i"an abstract type member `$name` that does not refine a member in its parent"
       case UnaccountedTypeParam(name) => i"an unaccounted type parameter `$name`"
@@ -5321,7 +5323,17 @@ object Types extends TypeUtils {
             else
               tycon.info match
                 case _: RealTypeBounds =>
-                  recAbstractTypeConstructor(pat)
+                  val tsym = tycon.typeSymbol
+                  if tsym.isOpaqueAlias
+                    && ctx.owner.isContainedIn(tsym.owner) // (1) opaque alias is known here, and
+                    && !ctx.owner.isAbstractOrAliasType    // (2) we are not in an embedded type
+                       // without (2), one cannot define a match type referring to an opaque type in the
+                       // scope of that opaque type. But that should be OK, we should simply not be
+                       // able to normalize such types in terms in the same scope.
+                  then
+                    MatchTypeCaseError.OpaqueAlias(tsym)
+                  else
+                    recAbstractTypeConstructor(pat)
                 case TypeAlias(tl @ HKTypeLambda(onlyParam :: Nil, resType: RefinedType)) =>
                   /* Unlike for eta-expanded classes, the typer does not automatically
                    * dealias poly type aliases to refined types. So we have to give them

tests/neg/named-tuples-strawman-2.check

@@ -0,0 +1,7 @@
+-- [E191] Type Error: tests/neg/named-tuples-strawman-2.scala:28:24 ----------------------------------------------------
+28 |    inline def toTuple: DropNames[NT] = x.asInstanceOf // error
+   |                        ^^^^^^^^^^^^^
+   |                   The match type contains an illegal case:
+   |                       case NamedTupleOps.NamedTuple[_, x] => x
+   |                   The pattern contains an opaque type alias `NamedTuple` in the scope where its alias is known.
+   |                   (this error can be ignored for now with `-source:3.3`)

tests/neg/named-tuples-strawman-2.scala

@@ -0,0 +1,29 @@
+import compiletime.*
+import compiletime.ops.int.*
+import compiletime.ops.boolean.!
+
+object NamedTupleDecomposition:
+  import NamedTupleOps.*
+
+  /** The names of the named tuple type `NT` */
+  type Names[NT <: AnyNamedTuple] <: Tuple = NT match
+    case NamedTuple[n, _] => n
+
+  /** The value types of the named tuple type `NT` */
+  type DropNames[NT <: AnyNamedTuple] <: Tuple = NT match
+    case NamedTuple[_, x] => x
+
+object NamedTupleOps:
+
+  opaque type AnyNamedTuple = Any
+
+  opaque type NamedTuple[N <: Tuple, +X <: Tuple] >: X <: AnyNamedTuple = X
+
+  export NamedTupleDecomposition.*
+
+  object NamedTuple:
+    def apply[N <: Tuple, X <: Tuple](x: X): NamedTuple[N, X] = x
+
+  extension [NT <: AnyNamedTuple](x: NT)
+    inline def toTuple: DropNames[NT] = x.asInstanceOf // error
+    inline def names: Names[NT] = constValueTuple[Names[NT]]

tests/neg/opaque-matches.check

@@ -0,0 +1,15 @@
+-- [E191] Type Error: tests/neg/opaque-matches.scala:4:18 --------------------------------------------------------------
+4 |  type Fst[X] = X match   // error
+  |                ^
+  |                The match type contains an illegal case:
+  |                    case NT.NT[a, b] => a
+  |                The pattern contains a type alias `NT`.
+  |                (this error can be ignored for now with `-source:3.3`)
+5 |    case NT[a, b] => a
+-- [E191] Type Error: tests/neg/opaque-matches.scala:10:30 -------------------------------------------------------------
+10 |  inline def snd[NT](nt: NT): NTDecomposition.Snd[NT] = // error
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^
+   |                           The match type contains an illegal case:
+   |                               case NT.NT[a, b] => b
+   |                           The pattern contains an opaque type alias `NT` in the scope where its alias is known.
+   |                           (this error can be ignored for now with `-source:3.3`)

tests/neg/opaque-matches.scala

@@ -0,0 +1,19 @@
+object NT:
+  opaque type NT[A, B] = B
+
+  type Fst[X] = X match   // error
+    case NT[a, b] => a
+
+  inline def fst[NT](nt: NT): Fst[NT] =
+    ???
+
+  inline def snd[NT](nt: NT): NTDecomposition.Snd[NT] = // error
+    ???
+
+object NTDecomposition:
+
+  type Snd[X] = X match
+    case NT.NT[a, b] => b
+
+
+

tests/run/named-tuples-strawman-2.scala

@@ -119,9 +119,11 @@ object TupleOps:
     for i <- toInclude.indices do
       arr(i) = xs.productElement(toInclude(i).asInstanceOf[Int]).asInstanceOf[Object]
     Tuple.fromArray(arr).asInstanceOf[Filter[X, P]]
+end TupleOps
 
 object NamedTupleDecomposition:
   import NamedTupleOps.*
+  import TupleOps.ConcatDistinct
 
   /** The names of the named tuple type `NT` */
   type Names[NT <: AnyNamedTuple] <: Tuple = NT match
@@ -131,21 +133,38 @@ object NamedTupleDecomposition:
   type DropNames[NT <: AnyNamedTuple] <: Tuple = NT match
     case NamedTuple[_, x] => x
 
+  extension [N1 <: Tuple, V1 <: Tuple](nt1: NamedTuple[N1, V1])
+    inline def updateWith[N2 <: Tuple, V2 <: Tuple](nt2: NamedTuple[N2, V2])
+      : UpdateWith[NamedTuple[N1, V1], NamedTuple[N2, V2]] =
+      val names = constValueTuple[ConcatDistinct[N1, N2]].toArray
+      val names2 = constValueTuple[N2].toArray
+      val values1 = NamedTupleOps.toTuple(nt1)
+      val values2 = nt2.toTuple
+      val values = new Array[Object](names.length)
+      values1.toArray.copyToArray(values)
+      for i <- 0 until values2.size do
+        val idx = names.indexOf(names2(i))
+        values(idx) = values2.productElement(i).asInstanceOf[Object]
+      Tuple.fromArray(values).asInstanceOf[UpdateWith[NamedTuple[N1, V1], NamedTuple[N2, V2]]]
+
+end NamedTupleDecomposition
+
 object NamedTupleOps:
   import TupleOps.*
 
   opaque type AnyNamedTuple = Any
 
   opaque type NamedTuple[N <: Tuple, +X <: Tuple] >: X <: AnyNamedTuple = X
 
-  export NamedTupleDecomposition.*
+  export NamedTupleDecomposition.{Names, DropNames}
 
   object NamedTuple:
     def apply[N <: Tuple, X <: Tuple](x: X): NamedTuple[N, X] = x
 
-  extension [NT <: AnyNamedTuple](x: NT)
-    inline def toTuple: DropNames[NT] = x.asInstanceOf
-    inline def names: Names[NT] = constValueTuple[Names[NT]]
+  extension [N <: Tuple, V <: Tuple](x: NamedTuple[N, V])
+    inline def toTuple: V = x.asInstanceOf
+
+    inline def names: N = constValueTuple[N]
 
   /** Internal use only: Merge names and value components of two named tuple to
    *  impement `UpdateWith`.
@@ -169,18 +188,8 @@ object NamedTupleOps:
   type UpdateWith[NT1 <: AnyNamedTuple, NT2 <: AnyNamedTuple] =
     Merge[ConcatDistinct[Names[NT1], Names[NT2]], DropNames[NT1], Names[NT2], DropNames[NT2]]
 
-  extension [NT1 <: AnyNamedTuple](nt1: NT1)
-    inline def updateWith[NT2 <: AnyNamedTuple](nt2: NT2): UpdateWith[NT1, NT2] =
-      val names = constValueTuple[ConcatDistinct[Names[NT1], Names[NT2]]].toArray
-      val names2 = constValueTuple[Names[NT2]].toArray
-      val values1 = nt1.toTuple
-      val values2 = nt2.toTuple
-      val values = new Array[Object](names.length)
-      values1.toArray.copyToArray(values)
-      for i <- 0 until values2.size do
-        val idx = names.indexOf(names2(i))
-        values(idx) = values2.productElement(i).asInstanceOf[Object]
-      Tuple.fromArray(values).asInstanceOf[UpdateWith[NT1, NT2]]
+  export NamedTupleDecomposition.updateWith
+end NamedTupleOps
 
 @main def Test =
   import TupleOps.*