[release-notes] F# in .NET 11 Preview 5

release-notes/11.0/preview/preview5/fsharp.md

@@ -0,0 +1,105 @@
+# F# in .NET 11 Preview 5 - Release Notes
+
+Here's a summary of what's new in F# in this Preview 5 release:
+
+- [Return-value attributes are preserved in more signatures](#return-value-attributes-are-preserved-in-more-signatures)
+- [Script files participate in parallel graph checking](#script-files-participate-in-parallel-graph-checking)
+- [Command-line warning controls apply during option parsing](#command-line-warning-controls-apply-during-option-parsing)
+- [F# 11 exception serialization preserves fields](#f-11-exception-serialization-preserves-fields)
+- [Bug fixes and other improvements](#bug-fixes-and-other-improvements)
+- [Community contributors](#community-contributors)
+
+## Return-value attributes are preserved in more signatures
+
+Return-value attributes are now routed to the return metadata slot for more F# member shapes. Prefix `[<return: ...>]` attributes on class members are emitted to IL, no longer conflict with member-level attributes under `AllowMultiple = false`, and attributes on unparenthesized tuple return types are no longer dropped ([dotnet/fsharp #19738](https://github.com/dotnet/fsharp/pull/19738), [dotnet/fsharp #19714](https://github.com/dotnet/fsharp/pull/19714)). Thank you [@edgarfgp](https://github.com/edgarfgp) for these contributions!
+
+```fsharp
+open System
+
+[<AttributeUsage(AttributeTargets.ReturnValue)>]
+type ReturnDescriptionAttribute(text: string) =
+    inherit Attribute()
+    member _.Text = text
+
+type Formatter =
+    [<return: ReturnDescription("normalized text")>]
+    member _.Normalize(value: string) = value.Trim()
+
+    static member Pair(value: string) : [<return: ReturnDescription("two values")>] string * string =
+        value, value.ToUpperInvariant()
+```
+
+## Script files participate in parallel graph checking
+
+Graph-based type checking now records modules that are implicitly provided by script files. Projects that include `.fsx` files can stay on the parallel checking path instead of falling back to sequential checking up to the last script file ([dotnet/fsharp #19649](https://github.com/dotnet/fsharp/pull/19649)). Thank you [@majocha](https://github.com/majocha) for this contribution!
+
+```fsharp
+// A.fsx
+let value = 41
+```
+
+```fsharp
+// B.fsx
+#load "A.fsx"
+
+let result = A.value + 1
+```
+
+## Command-line warning controls apply during option parsing
+
+`--nowarn` and `--warnaserror` now apply to warnings produced while the compiler parses command-line options. Option warnings now follow the same suppression and escalation rules as warnings reported later in compilation ([dotnet/fsharp #19776](https://github.com/dotnet/fsharp/pull/19776)).
+
+```console
+fsc --nowarn:75 --extraoptimizationloops:1 Program.fs
+```
+
+## F# 11 exception serialization preserves fields
+
+When targeting frameworks where `ISerializable` exception serialization is available, F# 11 now emits serialization members for user-defined exceptions with fields. The generated members store and restore those fields through `SerializationInfo`, so round trips preserve the exception payload ([dotnet/fsharp #19342](https://github.com/dotnet/fsharp/pull/19342), [dotnet/fsharp #19746](https://github.com/dotnet/fsharp/pull/19746)). This codegen is gated behind `--langversion:11`; earlier language versions keep the previous exception shape.
+
+```fsharp
+exception ParseFailed of line: int * message: string
+
+let error = ParseFailed(12, "Unexpected token")
+// On supported target frameworks, F# 11 emits serialization members
+// that preserve both the line number and message fields.
+```
+
+<!-- Filtered features (significant engineering work, but too niche for release notes):
+  - Parser grammar rebuild automation and emitted-IL baseline splitting: internal build/test infrastructure, not user-visible compiler behavior.
+  - Agentic workflow, PR tooling, WIF, SWIX, and dependency-update PRs: repository infrastructure or dependency flow only.
+  - Unfinished enum-pattern parser recovery and completion: useful editor-service work, but out of scope for these compiler/language/FSharp.Core/FSI notes.
+  - FSharp.Core documentation-only fixes for Array.exists2 and Async.StartChild: documentation corrections, not runtime or API behavior changes.
+-->
+
+## Bug fixes and other improvements
+
+### Compiler correctness
+
+- Custom attribute arguments now handle more valid inputs: empty arrays of user-defined types compile, non-empty arrays of unencodable types report FS3887 instead of an internal compiler error, and optional primitive value-type attribute constructor parameters get the correct default values ([dotnet/fsharp #19472](https://github.com/dotnet/fsharp/pull/19472), [dotnet/fsharp #19484](https://github.com/dotnet/fsharp/pull/19484)).
+- Attributes in `namespace rec` and `module rec` scopes now resolve opened namespaces before module attributes are checked ([dotnet/fsharp #19502](https://github.com/dotnet/fsharp/pull/19502)).
+- Nested inline SRTP functions with multiple overloads no longer hit internal error FS0073 during IL generation ([dotnet/fsharp #19710](https://github.com/dotnet/fsharp/pull/19710)). Thank you [@gusty](https://github.com/gusty)!
+- Pattern matching inside `seq`, list, and array comprehensions now narrows nullness correctly and avoids false-positive FS3261 warnings ([dotnet/fsharp #19743](https://github.com/dotnet/fsharp/pull/19743)).
+- Signature conformance now accepts an implementation member written as `member M(())` for a signature member written as `member M: unit -> unit` when the generated IL shape is the same ([dotnet/fsharp #19615](https://github.com/dotnet/fsharp/pull/19615)).
+
+### Diagnostics
+
+- Overload-resolution errors now underline the terminal method identifier instead of the entire object access and argument expression. Symbol-use ranges reported through name resolution now use the terminal identifier for the same cases ([dotnet/fsharp #19505](https://github.com/dotnet/fsharp/pull/19505)).
+- Parser diagnostics in debug builds no longer expose internal parser-state details, and related parse errors have clearer wording ([dotnet/fsharp #19730](https://github.com/dotnet/fsharp/pull/19730)). Thank you [@auduchinok](https://github.com/auduchinok)!
+- `use` bindings no longer produce an internal compiler error when a C#-style `Dispose` extension method is in scope alongside `IDisposable.Dispose` ([dotnet/fsharp #19568](https://github.com/dotnet/fsharp/pull/19568)).
+
+## Community contributors
+
+Thank you contributors! ❤️
+
+- [@auduchinok](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aauduchinok)
+- [@bbatsov](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Abbatsov)
+- [@edgarfgp](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aedgarfgp)
+- [@gusty](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Agusty)
+- [@majocha](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Amajocha)
+- [@quyentho](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aquyentho)
+
+F# updates:
+
+- [F# release notes](https://fsharp.github.io/fsharp-compiler-docs/release-notes/About.html)
+- [dotnet/fsharp repository](https://github.com/dotnet/fsharp)