Fix match type bounds checking problem

Author: odersky

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

@@ -145,22 +145,26 @@ object Checking {
     val checker = new TypeTraverser:
       def traverse(tp: Type) =
         tp match
-          case AppliedType(tycon, argTypes)
-          if !(tycon.typeSymbol.is(JavaDefined) && ctx.compilationUnit.isJava)
-            // Don't check bounds in Java units that refer to Java type constructors.
-            // Scala is not obliged to do Java type checking and in fact i17763 goes wrong
-            // if we attempt to check bounds of F-bounded mutually recursive Java interfaces.
-            // Do check all bounds in Scala units and those bounds in Java units that
-            // occur in applications of Scala type constructors.
-            && !isCaptureChecking || tycon.typeSymbol.is(CaptureChecked)
-            // Don't check bounds when capture checking type constructors that were not
-            // themselves capture checked. Since the type constructor could not foresee
-            // possible capture sets, it's better to be lenient for backwards compatibility.
-          =>
-            checkAppliedType(
-              untpd.AppliedTypeTree(TypeTree(tycon), argTypes.map(TypeTree(_)))
-                .withType(tp).withSpan(tpt.span.toSynthetic),
-              tpt)
+          case tp @ AppliedType(tycon, argTypes) =>
+            // Should the type be re-checked in the CC phase?
+            // Exempted are types that are not themselves capture-checked.
+            // Since the type constructor could not foresee possible capture sets,
+            // it's better to be lenient for backwards compatibility.
+            // Also exempted are match aliases. See tuple-ops.scala for an example that
+            // would fail otherwise.
+            def checkableUnderCC =
+              tycon.typeSymbol.is(CaptureChecked) && !tp.isMatchAlias
+            if !(tycon.typeSymbol.is(JavaDefined) && ctx.compilationUnit.isJava)
+                // Don't check bounds in Java units that refer to Java type constructors.
+                // Scala is not obliged to do Java type checking and in fact i17763 goes wrong
+                // if we attempt to check bounds of F-bounded mutually recursive Java interfaces.
+                // Do check all bounds in Scala units and those bounds in Java units that
+                // occur in applications of Scala type constructors.
+                && (!isCaptureChecking || checkableUnderCC) then
+              checkAppliedType(
+                untpd.AppliedTypeTree(TypeTree(tycon), argTypes.map(TypeTree(_)))
+                  .withType(tp).withSpan(tpt.span.toSynthetic),
+                tpt)
           case _ =>
         traverseChildren(tp)
     checker.traverse(tpt.tpe)

tests/pos-custom-args/captures/tuple-ops.scala

@@ -0,0 +1,13 @@
+sealed trait Tupp
+
+case object EmptyTupp extends Tupp
+type EmptyTupp = EmptyTupp.type
+infix case class `*::`[H, T <: Tupp](h: H, t: T) extends Tupp
+
+type Union[T <: Tupp] = T match
+  case EmptyTupp => Nothing
+  case h *:: t   => h | Union[t]
+
+type Map[T <: Tupp, F[_ <: Union[T]]] <: Tupp = T match
+  case EmptyTupp => EmptyTupp
+  case h *:: t   => F[h] *:: Map[t, F]