Fix C# cast simplification issue where removal of a cast can break an implicitly-typed array

src/EditorFeatures/Test2/Simplification/CastSimplificationTests.vb

@@ -4819,6 +4819,46 @@ class C
     }
 }
 ]]>
+</code>
+
+            Await TestAsync(input, expected)
+        End Function
+
+        <Fact, Trait(Traits.Feature, Traits.Features.Simplification)>
+        <WorkItem(6966, "https://github.com/dotnet/roslyn/issues/6966")>
+        Public Async Function TestCSharp_DontRemove_NecessaryCastToNullInImplicitlyTypedArray() As Task
+            Dim input =
+<Workspace>
+    <Project Language="C#" CommonReferences="true">
+        <Document><![CDATA[
+class C
+{
+    object M()
+    {
+        return new
+        {
+            Something = new[] { {|Simplify:(object)null|}, null, null, null }
+        };
+    }
+}
+]]>
+        </Document>
+    </Project>
+</Workspace>
+
+            Dim expected =
+<code><![CDATA[
+class C
+{
+    object M()
+    {
+        return new
+        {
+            Something = new[] { (object)null, null, null, null }
+        };
+    }
+}
+]]>
 </code>
 
             Await TestAsync(input, expected)

src/Workspaces/CSharp/Portable/Utilities/SpeculationAnalyzer.cs

@@ -426,6 +426,10 @@ protected override bool ReplacementChangesSemanticsForNodeLanguageSpecific(Synta
             {
                 return ReplacementBreaksInterpolation((InterpolationSyntax)currentOriginalNode, (InterpolationSyntax)currentReplacedNode);
             }
+            else if (currentOriginalNode.Kind() == SyntaxKind.ImplicitArrayCreationExpression)
+            {
+                return !TypesAreCompatible((ImplicitArrayCreationExpressionSyntax)currentOriginalNode, (ImplicitArrayCreationExpressionSyntax)currentReplacedNode);
+            }
 
             return false;
         }