Clarify diagnostic when constant pattern has non-constant conversion to input type

src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs

@@ -557,7 +557,16 @@ private BoundExpression BindExpressionForPatternContinued(
             {
                 if (constantValueOpt == null)
                 {
-                    diagnostics.Add(ErrorCode.ERR_ConstantExpected, patternExpression.Location);
+                    var strippedInputType = inputType.StrippedType();
+                    if (strippedInputType.Kind is not SymbolKind.ErrorType and not SymbolKind.DynamicType and not SymbolKind.TypeParameter &&
+                        strippedInputType.SpecialType is not SpecialType.System_Object and not SpecialType.System_ValueType)
+                    {
+                        diagnostics.Add(ErrorCode.ERR_ConstantValueOfTypeExpected, patternExpression.Location, strippedInputType);
+                    }
+                    else
+                    {
+                        diagnostics.Add(ErrorCode.ERR_ConstantExpected, patternExpression.Location);
+                    }
                     hasErrors = true;
                 }
                 else if (inputType.IsPointerType())

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7505,4 +7505,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_BadCaseInSwitchArm" xml:space="preserve">
     <value>A switch expression arm does not begin with a 'case' keyword.</value>
   </data>
+  <data name="ERR_ConstantValueOfTypeExpected" xml:space="preserve">
+    <value>A constant value of type '{0}' is expected</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2189,6 +2189,8 @@ internal enum ErrorCode
         ERR_BadStaticAfterUnsafe = 9133,
 
         ERR_BadCaseInSwitchArm = 9134,
+        ERR_ConstantValueOfTypeExpected = 9135,
+
         #endregion
 
         // Note: you will need to do the following after adding warnings:

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -2309,6 +2309,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.ERR_BadNullableReferenceTypeInUsingAlias:
                 case ErrorCode.ERR_BadStaticAfterUnsafe:
                 case ErrorCode.ERR_BadCaseInSwitchArm:
+                case ErrorCode.ERR_ConstantValueOfTypeExpected:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

src/Compilers/CSharp/Test/Emit/CodeGen/SwitchTests.cs

@@ -2623,10 +2623,10 @@ static void Main()
 }";
             var compilation = base.CreateCSharpCompilation(text);
             compilation.VerifyDiagnostics(
-                // (8,18): error CS0150: A constant value is expected
+                // (8,18): error CS9135: A constant value of type 'int' is expected
                 //             case i:
-                Diagnostic(ErrorCode.ERR_ConstantExpected, "i").WithLocation(8, 18)
-                );
+                Diagnostic(ErrorCode.ERR_ConstantValueOfTypeExpected, "i").WithArguments("int").WithLocation(8, 18)
+            );
         }
 
         [Fact, WorkItem(7625, "https://github.com/dotnet/roslyn/issues/7625")]