Add test for undefined behavior

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

@@ -5626,6 +5626,36 @@ public void Subsumption_Slice_03()
  );
  }
 
+ [Fact]
+ public void Subsumption_Slice_04()
+ {
+ var source = @"
+#nullable enable
+class C
+{
+ public int Length => 3;
+ public int this[int i] => 0;
+ public int[]? Slice(int i, int j) => null;
+
+ public static void Main()
+ {
+ switch (new C())
+ {
+ case [.. {}]:
+ break;
+ case [.. null]:
+ break;
+ }
+ }
+}
+";
+ var compilation = CreateCompilation(source, options: TestOptions.ReleaseExe);
+ compilation.VerifyEmitDiagnostics(
+ // (15,18): error CS8120: The switch case is unreachable. It has already been handled by a previous case or it is impossible to match.
+ // case [.. null]:
+ Diagnostic(ErrorCode.ERR_SwitchCaseSubsumed, "[.. null]").WithLocation(15, 18));
+ }
+
  [Fact]
  public void Exhaustiveness_01()
  {