Found TypeUtils tupleElementTypes, which is right

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -9,6 +9,7 @@ import Symbols._, Contexts._, Types._, StdNames._, NameOps._
 import util.Spans._
 import typer.Applications.*
 import SymUtils._
+import TypeUtils.*
 import Flags._, Constants._
 import Decorators._
 import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
@@ -332,7 +333,7 @@ object PatternMatcher {
           ref(defn.RuntimeTuplesModule)
             .select(defn.RuntimeTuples_apply)
             .appliedTo(receiver, Literal(Constant(i)))
-            .cast(indexTupleType(tuple, i).ensuring(_ != NoType, i"Failed to find type at index $i in $tuple"))
+            .cast(tuple.tupleElementTypes.applyOrElse(i, (_: Int) => NoType).ensuring(_ != NoType, i"Failed to find type at index $i in $tuple (${tree.sourcePos})"))
 
         if (isSyntheticScala2Unapply(unapp.symbol) && caseAccessors.length == args.length)
           def tupleSel(sym: Symbol) = ref(scrutinee).select(sym)

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

@@ -207,11 +207,6 @@ object Applications {
     end tupleFold
     tupleFold(Nil, tp).reverse
 
-  /** The component type of a generic tuple at index `idx`.  For example, `indexTupleType(A *: B *: EmptyTuple, 1) = B`. */
-  def indexTupleType(tp: Type, idx: Int)(using Context): Type = tp.widen match
-    case AppliedType(tycon, List(l, r)) if tycon.typeSymbol == defn.PairClass => if idx == 0 then l else indexTupleType(r, idx - 1)
-    case t                                                                    => NoType
-
   def wrapDefs(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree =
     if (defs != null && defs.nonEmpty) tpd.Block(defs.toList, tree) else tree