solves dotc assertion error on (non-sensical?) parameter type. #13769

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -442,6 +442,9 @@ object Parsers {
     def convertToParam(tree: Tree, mods: Modifiers, expected: String = "formal parameter"): ValDef = tree match {
       case id @ Ident(name) =>
         makeParameter(name.asTermName, TypeTree(), mods, isBackquoted = isBackquoted(id)).withSpan(tree.span)
+      case Typed(_, tpt: TypeBoundsTree) =>
+        syntaxError(s"not a legal $expected", tree.span)
+        makeParameter(nme.ERROR, tree, mods)
       case Typed(id @ Ident(name), tpt) =>
         makeParameter(name.asTermName, tpt, mods, isBackquoted = isBackquoted(id)).withSpan(tree.span)
       case Typed(Splice(Ident(name)), tpt) =>

tests/neg/i13769.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/i13769.scala:2:18 ----------------------------------------------------------------------------------
+2 |val te = tup.map((x: _ <: Int) => List(x)) // error // error
+  |                  ^^^^^^^^^^^
+  |                  not a legal formal parameter
+-- [E006] Not Found Error: tests/neg/i13769.scala:2:39 -----------------------------------------------------------------
+2 |val te = tup.map((x: _ <: Int) => List(x)) // error // error
+  |                                       ^
+  |                                       Not found: x
+
+longer explanation available when compiling with `-explain`

tests/neg/i13769.scala

@@ -0,0 +1,2 @@
+val tup = (1, "s")
+val te = tup.map((x: _ <: Int) => List(x)) // error // error