Mark default value operations as having no side effects

src/EditorFeatures/CSharpTest/RemoveUnusedParametersAndValues/RemoveUnusedValueAssignmentTests.cs

@@ -268,6 +268,31 @@ int M()
 }", optionName);
         }
 
+        [Theory, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedValues)]
+        [InlineData(nameof(PreferDiscard))]
+        [InlineData(nameof(PreferUnusedLocal))]
+        public async Task Initialization_NonConstantValue_DefaultExpression(string optionName)
+        {
+            await TestInRegularAndScriptAsync(
+@"struct C
+{
+    C M()
+    {
+        C [|c|] = default(C);
+        c = new C();
+        return c;
+    }
+}",
+@"struct C
+{
+    C M()
+    {
+        C c = new C();
+        return c;
+    }
+}", optionName);
+        }
+
         [Theory, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedValues)]
         [InlineData(nameof(PreferDiscard))]
         [InlineData(nameof(PreferUnusedLocal))]

src/Features/Core/Portable/RemoveUnusedParametersAndValues/AbstractRemoveUnusedParametersAndValuesDiagnosticAnalyzer.SymbolStartAnalyzer.BlockAnalyzer.cs

@@ -405,16 +405,20 @@ bool IsRemovableAssignmentWithoutSideEffects(IOperation unusedSymbolWriteOperati
 
                             switch (assignment.Value.Kind)
                             {
-                                // Parameter/local references have no side effects and can be removed.
                                 case OperationKind.ParameterReference:
                                 case OperationKind.LocalReference:
+                                    // Parameter/local references have no side effects and can be removed.
                                     return true;
 
-                                // Field references with null instance (static fields) or 'this' or 'Me' instance can
-                                // have no side effects and can be removed.
                                 case OperationKind.FieldReference:
+                                    // Field references with null instance (static fields) or 'this' or 'Me' instance can
+                                    // have no side effects and can be removed.
                                     var fieldReference = (IFieldReferenceOperation)assignment.Value;
                                     return fieldReference.Instance == null || fieldReference.Instance.Kind == OperationKind.InstanceReference;
+
+                                case OperationKind.DefaultValue:
+                                    // default value expressions have no side-effects.
+                                    return true;
                             }
                         }
                         else if (unusedSymbolWriteOperation.Parent is IIncrementOrDecrementOperation)