When assigning type cases, substitute any instances of type parameters for the applied arguments

Author: dwijnand

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

@@ -401,15 +401,19 @@ 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 params = getParams(new mutable.ListBuffer[TypeSymbol](), pat).toList
+        pat.tpe match
+          case AppliedType(tycon, args) =>
+            val tparams = tycon.typeParamSymbols
+            for param <- params do param.info = param.info.subst(tparams, args))
+          case _ =>
+        HKTypeLambda.fromParams(params, defn.MatchCase(pat.tpe, body.tpe))
       }
       else body.tpe
     tree.withType(ownType)