Don't suggest that casts are unnecessary when no conversion exists.

src/EditorFeatures/CSharpTest/Diagnostics/RemoveUnnecessaryCast/RemoveUnnecessaryCastTests.cs

@@ -4492,7 +4492,7 @@ void M()
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)]
-        public async Task DontOfferToRemoveCastFormObjectToStruct()
+        public async Task DontOfferToRemoveCastWhereNoConversionExists()
         {
             await TestMissingInRegularAndScriptAsync(
                 @"

src/Workspaces/CSharp/Portable/Extensions/CastExpressionSyntaxExtensions.cs

@@ -393,6 +393,12 @@ public static bool IsUnnecessaryCast(this CastExpressionSyntax cast, SemanticMod
             var expressionToCastType = semanticModel.ClassifyConversion(cast.SpanStart, cast.Expression, castType, isExplicitInSource: true);
             var outerType = GetOuterCastType(cast, semanticModel, out var parentIsOrAsExpression) ?? castTypeInfo.ConvertedType;
 
+            // If there is no conversion then there will be a compiler error, but offering the code fix before that is confusing, so jump out.
+            if (!expressionToCastType.Exists)
+            {
+                return false;
+            }
+            
             // Simple case: If the conversion from the inner expression to the cast type is identity,
             // the cast can be removed.
             if (expressionToCastType.IsIdentity)