Fix possible crash in Desugar

This might fix #19560. [Cherry-picked 6e31578]

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1502,15 +1502,17 @@ object desugar {
  * skipping elements that are not convertible.
  */
  def patternsToParams(elems: List[Tree])(using Context): List[ValDef] =
- def toParam(elem: Tree, tpt: Tree): Tree =
+ def toParam(elem: Tree, tpt: Tree, span: Span): Tree =
  elem match
- case Annotated(elem1, _) => toParam(elem1, tpt)
- case Typed(elem1, tpt1) => toParam(elem1, tpt1)
- case Ident(id: TermName) => ValDef(id, tpt, EmptyTree).withFlags(Param)
+ case Annotated(elem1, _) => toParam(elem1, tpt, span)
+ case Typed(elem1, tpt1) => toParam(elem1, tpt1, span)
+ case Ident(id: TermName) => ValDef(id, tpt, EmptyTree).withFlags(Param).withSpan(span)
  case _ => EmptyTree
- elems.map(param => toParam(param, TypeTree()).withSpan(param.span)).collect {
- case vd: ValDef => vd
- }
+ elems
+ .map: param =>
+ toParam(param, TypeTree(), param.span)
+ .collect:
+ case vd: ValDef => vd
 
  def makeContextualFunction(formals: List[Tree], body: Tree, erasedParams: List[Boolean])(using Context): Function = {
  val mods = Given