test: add in a regression test for #11706 (#17499)

[skip community_build]

closes #11706

Author: nicolasstucki

tests/run/i11706.check

@@ -0,0 +1,2 @@
+3
+3

tests/run/i11706.scala

@@ -0,0 +1,30 @@
+// https://github.com/lampepfl/dotty/issues/11706
+import scala.compiletime.erasedValue
+
+object Obj:
+
+  inline def length[Tuple]: Int = loop[Tuple]
+
+  private inline def loop[Tuple]: Int =
+    inline erasedValue[Tuple] match
+      case _: EmptyTuple     => 0
+      case _: (head *: tail) => 1 + loop[tail]
+
+end Obj
+
+// Same code, but in a trait instead of an object
+trait Trait:
+
+  inline def length[Tuple]: Int = loop[Tuple]
+
+  private inline final def loop[Tuple]: Int =
+    inline erasedValue[Tuple] match
+      case _: EmptyTuple     => 0
+      case _: (head *: tail) => 1 + loop[tail]
+
+end Trait
+
+@main def Test() =
+  println(Obj.length[(Int, Int, String)]) // OK
+  new Trait:
+    println(length[(Int, Int, String)])