Fix bounds of match type cases

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

@@ -401,15 +401,25 @@ trait TypeAssigner {
   def assignType(tree: untpd.CaseDef, pat: Tree, body: Tree)(using Context): CaseDef = {
     val ownType =
       if (body.isType) {
-        val params = new TreeAccumulator[mutable.ListBuffer[TypeSymbol]] {
+        val getParams = new TreeAccumulator[mutable.ListBuffer[TypeSymbol]] {
           def apply(ps: mutable.ListBuffer[TypeSymbol], t: Tree)(using Context) = t match {
             case t: Bind if t.symbol.isType => foldOver(ps += t.symbol.asType, t)
             case _ => foldOver(ps, t)
           }
         }
-        HKTypeLambda.fromParams(
-          params(new mutable.ListBuffer[TypeSymbol](), pat).toList,
-          defn.MatchCase(pat.tpe, body.tpe))
+        val params1 = getParams(new mutable.ListBuffer[TypeSymbol](), pat).toList
+        val params2 = pat.tpe match
+          case AppliedType(tycon, args) =>
+            val tparams = tycon.typeParamSymbols
+            params1.mapconserve { param =>
+              val info1 = param.info
+              val info2 = info1.subst(tparams, args)
+              if info2 eq info1 then param else param.copy(info = info2).asType
+            }
+          case _ => params1
+        val matchCase1 = defn.MatchCase(pat.tpe, body.tpe)
+        val matchCase2 = if params2 eq params1 then matchCase1 else matchCase1.substSym(params1, params2)
+        HKTypeLambda.fromParams(params2, matchCase2)
       }
       else body.tpe
     tree.withType(ownType)

tests/pos/i14477.scala

@@ -0,0 +1,9 @@
+sealed trait Foo[A, X <: A]
+
+type FooX[F] = F match {
+  case Foo[a, x] => x
+}
+
+type MyFoo = Foo[String, "hello"]
+
+val hello: FooX[MyFoo] = "hello"