Add regression test for issue 18493

[Cherry-picked b93763a]

Author: jchyb

tests/neg/18493.check

@@ -0,0 +1,8 @@
+-- [E030] Match case Unreachable Error: tests/neg/18493.scala:6:9 ------------------------------------------------------
+6 |    case "abc" => // error
+  |         ^^^^^
+  |         Unreachable case
+-- [E030] Match case Unreachable Error: tests/neg/18493.scala:12:9 -----------------------------------------------------
+12 |    case "abc" => // error
+   |         ^^^^^
+   |         Unreachable case

tests/neg/18493.scala

@@ -0,0 +1,14 @@
+//> using options -Werror
+object PartialFunctionNoWarning {
+  // nice warning
+  "abc" match {
+    case "abc" =>
+    case "abc" => // error
+  }
+  
+  // no warnings
+  val pf: PartialFunction[String, Unit] = {
+    case "abc" =>
+    case "abc" => // error
+  }
+}