Fix a bug in code gen for switch on byte or ushort (#18521)

Fixes #18188

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

@@ -3014,6 +3014,72 @@ void M(bool b)
                 Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion1, "b").WithArguments("switch on boolean type", "2"));
         }
 
+        [Fact]
+        public void SmallUnsignedEdgeCase01()
+        {
+            var source = @"
+using System;
+class C
+{
+    static void Main(string[] args)
+    {
+        RunTest(126);
+        RunTest(127);
+        RunTest(128);
+        RunTest(129);
+
+        void RunTest(byte testByte)
+        {
+            switch (testByte)
+            {
+                case 127: // 0111 1111
+                case 128: // 1000 0000
+                    Console.Write(0);
+                    break;
+                default:
+                    Console.Write(1);
+                    break;
+            }
+        }
+    }
+}";
+            var comp = CompileAndVerify(source, expectedOutput: @"1001");
+            comp.VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void SmallUnsignedEdgeCase02()
+        {
+            var source = @"
+using System;
+class C
+{
+    static void Main(string[] args)
+    {
+        RunTest(32766);
+        RunTest(32767);
+        RunTest(32768);
+        RunTest(32769);
+
+        void RunTest(ushort testUshort)
+        {
+            switch (testUshort)
+            {
+                case 32767: // 0111 1111 1111 1111
+                case 32768: // 1000 0000 0000 0000
+                    Console.Write(0);
+                    break;
+                default:
+                    Console.Write(1);
+                    break;
+            }
+        }
+    }
+}";
+            var comp = CompileAndVerify(source, expectedOutput: @"1001");
+            comp.VerifyDiagnostics();
+        }
+
         #endregion
     }
 }

src/Compilers/Core/Portable/CodeGen/SwitchIntegralJumpTableEmitter.cs

@@ -466,7 +466,20 @@ private void EmitRangeCheckedBranch(ConstantValue startConstant, ConstantValue e
             }
             else
             {
-                _builder.EmitIntConstant(endConstant.Int32Value - startConstant.Int32Value);
+                int Int32Value(ConstantValue value)
+                {
+                    // ConstantValue does not correctly convert byte and ushort values to int.
+                    // It sign extends them rather than padding them. We compensate for that here.
+                    // See also https://github.com/dotnet/roslyn/issues/18579
+                    switch (value.Discriminator)
+                    {
+                        case ConstantValueTypeDiscriminator.Byte: return value.ByteValue;
+                        case ConstantValueTypeDiscriminator.UInt16: return value.UInt16Value;
+                        default: return value.Int32Value;
+                    }
+                }
+
+                _builder.EmitIntConstant(Int32Value(endConstant) - Int32Value(startConstant));
             }
 
             _builder.EmitBranch(ILOpCode.Ble_un, targetLabel, ILOpCode.Bgt_un);