Skip union case tester properties in API docs

RELEASE_NOTES.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 * Fix tooltip not being interactive: moving the mouse from a code token into its tooltip now keeps the tooltip open, allowing users to select and copy the tooltip text. [#949](https://github.com/fsprojects/FSharp.Formatting/issues/949)
+* The compiler-generated `IsCase` union case tester properties are no longer listed as members of a union type, and `FsDocsWarnOnMissingDocs` no longer reports `FD0001` for them: they cannot carry XML documentation. [#1313](https://github.com/fsprojects/FSharp.Formatting/issues/1313)
 * `fsdocs watch` rebuilds a page when a file its script depends on changes, following `#load` transitively and `#r` to local files, wherever those files are (a dot folder, outside the input folder). The directives are read from the syntax tree, so a `#load` in a comment does not count. [#1309](https://github.com/fsprojects/FSharp.Formatting/issues/1309)
 
 ## [23.0.0-alpha.1] - 2026-09-08

src/FSharp.Formatting.ApiDocs/SymbolReader.fs

@@ -836,13 +836,19 @@ module internal SymbolReader =
                 ctx.WarnOnMissingDocs
             ))
 
+    /// Members the compiler synthesizes for the user: compiler-generated members and the
+    /// <c>IsCase</c> union case tester properties. They cannot carry XML documentation, so they
+    /// are neither documented nor reported as missing documentation.
+    let isSynthesizedMember (v: FSharpMemberOrFunctionOrValue) =
+        v.IsCompilerGenerated || v.IsUnionCaseTester
+
     /// Reads all members in a sequence without filtering out any by kind.
     let readAllMembers ctx entityUrl kind (members: FSharpMemberOrFunctionOrValue seq) =
         members
         |> Seq.choose (fun v ->
             if
                 checkAccess ctx v.Accessibility
-                && not v.IsCompilerGenerated
+                && not (isSynthesizedMember v)
                 && not v.IsPropertyGetterMethod
                 && not v.IsPropertySetterMethod
                 && not v.IsEventAddMethod
@@ -858,7 +864,7 @@ module internal SymbolReader =
     let readMembers ctx entityUrl kind (entity: FSharpEntity) cond =
         entity.MembersFunctionsAndValues
         |> Seq.choose (fun v ->
-            if checkAccess ctx v.Accessibility && not v.IsCompilerGenerated && cond v then
+            if checkAccess ctx v.Accessibility && not (isSynthesizedMember v) && cond v then
                 tryReadMember ctx entityUrl kind v
             else
                 None)
@@ -1053,7 +1059,7 @@ module internal SymbolReader =
                                     bdef.MembersFunctionsAndValues
                                     |> Seq.filter (fun v ->
                                         checkAccess ctx v.Accessibility
-                                        && not v.IsCompilerGenerated
+                                        && not (isSynthesizedMember v)
                                         && not v.IsOverrideOrExplicitInterfaceImplementation
                                         && not v.IsEventAddMethod
                                         && not v.IsEventRemoveMethod
@@ -1088,7 +1094,7 @@ module internal SymbolReader =
                 getMembers typ
                 |> Seq.filter (fun v ->
                     checkAccess ctx v.Accessibility
-                    && not v.IsCompilerGenerated
+                    && not (isSynthesizedMember v)
                     && not v.IsOverrideOrExplicitInterfaceImplementation
                     && not v.IsEventAddMethod
                     && not v.IsEventRemoveMethod

tests/FSharp.ApiDocs.Tests/ApiDocsTests.fs

@@ -302,6 +302,10 @@ let ``ApiDocs works on two sample F# assemblies`` (format: OutputFormat) =
     files.[(sprintf "fslib-union.%s" format.Extension)]
     |> shouldContainText "Hello of int"
 
+    // The compiler-generated `IsCase` union case testers are not documented (#1313)
+    files.[(sprintf "fslib-union.%s" format.Extension)]
+    |> shouldNotContainText "IsHello"
+
     files.[(sprintf "fslib.%s" format.Extension)]
     |> shouldContainText "Sample class"