Fix #12974: Add test case

Author: OlivierBlanvillain

tests/neg/12974.scala

@@ -0,0 +1,35 @@
+package example
+
+object RecMap {
+
+  object Record {
+    // use this scope to bound who can see inside the opaque type
+    opaque type Rec[A <: Tuple] = Map[String, Any]
+
+    object Rec {
+      type HasKey[A <: Tuple, K] =
+        A match
+          case (K, t) *: _ => t
+          case _ *: t => HasKey[t, K]
+
+      val empty: Rec[EmptyTuple] = Map.empty
+
+      extension [A <: Tuple](toMap: Rec[A])
+        def fetch[K <: String & Singleton](key: K): HasKey[A, K] =
+          toMap(key).asInstanceOf[HasKey[A, K]]
+    }
+  }
+
+  def main(args: Array[String]) =
+    import Record._
+
+    val foo: Any = Rec.empty.fetch("foo") // error
+    //                            ^
+    // Match type reduction failed since selector  EmptyTuple.type
+    // matches none of the cases
+    //
+    //     case (("foo" : String), t) *: _ => t
+    //     case _ *: t => example.RecMap.Record.Rec.HasKey[t, ("foo" : String)]
+
+  end main
+}