Add test

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

@@ -5596,6 +5596,36 @@ interface IInner
  );
  }
 
+ [Fact]
+ public void Subsumption_Slice_03()
+ {
+ var source = @"
+using System;
+Span<int> s = default;
+switch (s) {
+ case [..[1],2,3]:
+ case [1,2,3]: // 1
+ break;
+}
+
+int[] a = default;
+switch (a) {
+ case [..[1],2,3]:
+ case [1,2,3]: // 2
+ break;
+}
+";
+ var comp = CreateCompilationWithIndexAndRangeAndSpan(new[] { source, TestSources.GetSubArray }, options: TestOptions.DebugExe);
+ comp.VerifyEmitDiagnostics(
+ // (6,10): error CS8120: The switch case is unreachable. It has already been handled by a previous case or it is impossible to match.
+ // case [1,2,3]: // 1
+ Diagnostic(ErrorCode.ERR_SwitchCaseSubsumed, "[1,2,3]").WithLocation(6, 10),
+ // (13,10): error CS8120: The switch case is unreachable. It has already been handled by a previous case or it is impossible to match.
+ // case [1,2,3]: // 2
+ Diagnostic(ErrorCode.ERR_SwitchCaseSubsumed, "[1,2,3]").WithLocation(13, 10)
+ );
+ }
+
  [Fact]
  public void Exhaustiveness_01()
  {