Add a refactoring between F# and .NET-compatible optional parameters

docs/release-notes/.VisualStudio/18.vNext.md

@@ -1,5 +1,6 @@
 ### Added
 
+* Refactoring to switch a member's optional parameter between F# `?x: T` and the .NET-compatible `[<Optional; DefaultParameterValue(c)>] x: T`, moving the default between `defaultArg x c` in the body and the attribute; F# call sites keep working unchanged. From F# 10 the .NET form also converts back to `[<Struct>] ?x: T` with `defaultValueArg x c`. ([PR #20547](https://github.com/dotnet/fsharp/pull/20547))
 * Code-fixes for FS3888 (compiler-semantic attribute on the `.fs` but not the `.fsi`): copy the attribute into the `.fsi`, or remove it from the `.fs`. ([Issue #19560](https://github.com/dotnet/fsharp/issues/19560), [PR #19880](https://github.com/dotnet/fsharp/pull/19880))
 * Expand `<inheritdoc/>` in IDE tooltips, completion, and signature help, inheriting XML documentation from base classes, interfaces, overridden members, and constructors. ([Issue #19175](https://github.com/dotnet/fsharp/issues/19175), [PR #19188](https://github.com/dotnet/fsharp/pull/19188))
 

vsintegration/src/FSharp.Editor/FSharp.Editor.fsproj

@@ -105,6 +105,7 @@
     <Compile Include="Refactor\ChangeTypeofWithNameToNameofExpression.fs" />
     <Compile Include="Refactor\AddExplicitTypeToParameter.fs" />
     <Compile Include="Refactor\ChangeDerefToValueRefactoring.fs" />
+    <Compile Include="Refactor\ConvertOptionalParameterDefaultValue.fs" />
     <Compile Include="CodeFixes\IFSharpCodeFix.fs" />
     <Compile Include="CodeFixes\CodeFixHelpers.fs" />
     <Compile Include="CodeFixes\ChangeEqualsInFieldTypeToColon.fs" />

vsintegration/src/FSharp.Editor/FSharp.Editor.resx

@@ -368,4 +368,13 @@ Use live (unsaved) buffers for analysis</value>
   <data name="ReturnsHeader" xml:space="preserve">
     <value>Returns:</value>
   </data>
+  <data name="UseDotNetOptionalParameter" xml:space="preserve">
+    <value>Use [&lt;Optional; DefaultParameterValue&gt;] for optional parameter</value>
+  </data>
+  <data name="UseFSharpOptionalParameter" xml:space="preserve">
+    <value>Use F# '?' optional parameter</value>
+  </data>
+  <data name="UseFSharpStructOptionalParameter" xml:space="preserve">
+    <value>Use F# '[&lt;Struct&gt;] ?' optional parameter</value>
+  </data>
 </root>

vsintegration/tests/FSharp.Editor.Tests/FSharp.Editor.Tests.fsproj

@@ -74,6 +74,7 @@
     <Compile Include="CodeFixes\AddMissingSeqTests.fs" />
     <Compile Include="Refactors\RefactorTestFramework.fs" />
     <Compile Include="Refactors\AddReturnTypeTests.fs" />
+    <Compile Include="Refactors\ConvertOptionalParameterDefaultValueTests.fs" />
     <Compile Include="Hints\HintTestFramework.fs" />
     <Compile Include="Hints\OptionParserTests.fs" />
     <Compile Include="Hints\InlayParameterNameHintTests.fs" />