Ensure pt span exists in implicitParams migration (#23372)

Fixes #23022

Author: mbovel

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

@@ -131,7 +131,7 @@ trait Migrations:
   /** Report implicit parameter lists and rewrite implicit parameter list to contextual params */
   def implicitParams(tree: Tree, tp: MethodOrPoly, pt: FunProto)(using Context): Unit =
     val mversion = mv.ImplicitParamsWithoutUsing
-    if tp.companion == ImplicitMethodType && pt.applyKind != ApplyKind.Using && pt.args.nonEmpty then
+    if tp.companion == ImplicitMethodType && pt.applyKind != ApplyKind.Using && pt.args.nonEmpty && pt.args.head.span.exists then
       // The application can only be rewritten if it uses parentheses syntax.
       // See issue #22927 and related tests.
       val hasParentheses = checkParentheses(tree, pt)
@@ -160,8 +160,10 @@ trait Migrations:
   end implicitParams
 
   private def checkParentheses(tree: Tree, pt: FunProto)(using Context): Boolean =
-    ctx.source.content
-      .slice(tree.span.end, pt.args.head.span.start)
+    val ptSpan = pt.args.head.span
+    ptSpan.exists
+    && ctx.source.content
+      .slice(tree.span.end, ptSpan.start)
       .exists(_ == '(')
 
   private def patchImplicitParams(tree: Tree, pt: FunProto)(using Context): Unit =

tests/pos/i23022.scala

@@ -0,0 +1,12 @@
+trait ExtractorWithImplicit:
+
+  object Yikes:
+    def unapply(implicit M: String): Option[Any] = ???
+
+  def expand: Any =
+    given String = "Hey"
+    "Wut" match
+      case Yikes(_) => ???
+      case _       => ???
+
+