Add regression test for scala#23350

[Cherry-picked 3c892d3]

Author: mbovel

tests/neg/i23350.check

@@ -0,0 +1,46 @@
+-- [E120] Naming Error: tests/neg/i23350.scala:8:7 ---------------------------------------------------------------------
+8 |object D extends A: // error
+  |       ^
+  |       Name clash between defined and inherited member:
+  |       def apply(p: A.this.Props): Unit in class A at line 5 and
+  |       def apply(a: UndefOr2[String]): Unit in object D at line 10
+  |       have the same type (a: Object): Unit after erasure.
+  |
+  |       Consider adding a @targetName annotation to one of the conflicting definitions
+  |       for disambiguation.
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  |
+  | As part of the Scala compilation pipeline every type is reduced to its erased
+  | (runtime) form. In this phase, among other transformations, generic parameters
+  | disappear and separate parameter-list boundaries are flattened.
+  |
+  | For example, both `f[T](x: T)(y: String): Unit` and `f(x: Any, z: String): Unit`
+  | erase to the same runtime signature `f(x: Object, y: String): Unit`. Note that
+  | parameter names are irrelevant.
+  |
+  | In your code the two declarations
+  |
+  |   def apply(p: A.this.Props): Unit
+  |   def apply(a: UndefOr2[String]): Unit
+  |
+  | erase to the identical signature
+  |
+  |   (a: Object): Unit
+  |
+  | so the compiler cannot keep both: the generated bytecode symbols would collide.
+  |
+  | To fix this error, you need to disambiguate the two definitions. You can either:
+  |
+  | 1. Rename one of the definitions, or
+  | 2. Keep the same names in source but give one definition a distinct
+  |    bytecode-level name via `@targetName` for example:
+  |
+  |       @targetName("apply_2")
+  |       def apply(a: UndefOr2[String]): Unit
+  |
+  | Choose the `@targetName` argument carefully: it is the name that will be used
+  | when calling the method externally, so it should be unique and descriptive.
+  |         
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/i23350.scala

@@ -0,0 +1,10 @@
+//> using options -explain
+
+abstract class A:
+  type Props
+  def apply(p: Props) = ()
+
+type UndefOr2[A] = A | Unit
+object D extends A: // error
+  case class Props()
+  def apply(a: UndefOr2[String]) = ()