[Repo Assist] Warn on unrecognized .editorconfig property values instead of silently ignoring them

CHANGELOG.md

@@ -17,6 +17,7 @@
 - Open-ended expressions (lambda, if-then-else, match, ...) in non-last positions of infix, tuple, list/array, and record expressions now stay multiline to preserve semantics. [#3279](https://github.com/fsprojects/fantomas/issues/3279)
 - Lambda in tuple in list on single line changes code meaning. [#3278](https://github.com/fsprojects/fantomas/issues/3278)
 - Custom operator applied to lambda collapses to single line changing semantics. [#3274](https://github.com/fsprojects/fantomas/issues/3274)
+- Unrecognized `.editorconfig` property values now emit a warning to stderr instead of being silently ignored. [#1236](https://github.com/fsprojects/fantomas/issues/1236)
 
 ## [8.0.0-alpha-007] - 2026-03-10
 

src/Fantomas.Tests/EditorConfigurationTests.fs

@@ -543,3 +543,33 @@ fsharp_experimental_elmish = true
     let config = EditorConfig.readConfiguration fsharpFile.FSharpFile
 
     Assert.That(config.ExperimentalElmish, Is.True)
+
+[<Test>]
+let ``invalid editorconfig value emits warning to stderr and uses default`` () =
+    let rootDir = tempName ()
+
+    let editorConfig =
+        """
+[*.fs]
+fsharp_experimental_elmish = not_a_bool
+"""
+
+    use configFixture =
+        new ConfigurationFile(defaultConfig, rootDir, content = editorConfig)
+
+    use fsharpFile = new FSharpFile(rootDir)
+
+    let capturedErr = new System.IO.StringWriter()
+    let originalErr = Console.Error
+    Console.SetError(capturedErr)
+
+    let config =
+        try
+            EditorConfig.readConfiguration fsharpFile.FSharpFile
+        finally
+            Console.SetError(originalErr)
+
+    let output = capturedErr.ToString()
+    Assert.That(output, Does.Contain("fsharp_experimental_elmish"))
+    Assert.That(output, Does.Contain("not_a_bool"))
+    config.ExperimentalElmish == defaultConfig.ExperimentalElmish

src/Fantomas/EditorConfig.fs

@@ -88,7 +88,12 @@ let parseOptionsFromEditorConfig
         | true, MultilineFormatterType mft -> box mft
         | true, EndOfLineStyle eol -> box eol
         | true, BracketStyle bs -> box bs
-        | _ -> defaultValue)
+        | false, _ -> defaultValue
+        | true, invalidValue ->
+            eprintfn
+                $"warning: unrecognized value '%s{invalidValue}' for property '%s{editorConfigName}' in .editorconfig, using default."
+
+            defaultValue)
     |> fun newValues ->
 
         let formatConfigType = FormatConfig.Default.GetType()