Use union of cases as default bound of match types

Author: OlivierBlanvillain

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1498,7 +1498,10 @@ class Typer extends Namer
     val sel1 = typed(tree.selector)
     val pt1 = if (bound1.isEmpty) pt else bound1.tpe
     val cases1 = tree.cases.mapconserve(typedTypeCase(_, sel1.tpe, pt1))
-    assignType(cpy.MatchTypeTree(tree)(bound1, sel1, cases1), bound1, sel1, cases1)
+    val bound2 =
+      if (tree.bound.isEmpty) TypeTree(cases1.map(_.body.tpe).reduce(_ | _))
+      else bound1
+    assignType(cpy.MatchTypeTree(tree)(bound2, sel1, cases1), bound2, sel1, cases1)
   }
 
   def typedByNameTypeTree(tree: untpd.ByNameTypeTree)(implicit ctx: Context): ByNameTypeTree = {

Diff for: tests/pos/i90.scala

@@ -0,0 +1,5 @@
+object Test {
+  type F[X] = X match { case 10 => "0" case 11 => "1" }
+
+  implicitly[F[Any] <:< String]
+}