[release-notes] F# in .NET 11 Preview 4

release-notes/11.0/preview/preview4/fsharp.md

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+# F# in .NET 11 Preview 4 - Release Notes
+
+Here's a summary of what's new in F# in this Preview 4 release:
+
+- [Improved diagnostic messages](#improved-diagnostic-messages)
+- [Codegen correctness — fewer runtime crashes from valid F# code](#codegen-correctness--fewer-runtime-crashes-from-valid-f-code)
+- [Signature generation roundtrips cleanly](#signature-generation-roundtrips-cleanly)
+- [Missing implementations for C# `abstract override` are now caught at compile time](#missing-implementations-for-c-abstract-override-are-now-caught-at-compile-time)
+- [Faster IDE responsiveness with generative type providers](#faster-ide-responsiveness-with-generative-type-providers)
+- [Bug fixes and other improvements](#bug-fixes-and-other-improvements)
+
+## Improved diagnostic messages
+
+A batch of long-standing usability issues with compiler diagnostics has been addressed. The focus is clearer wording, actionable hints, and catching common beginner mistakes that previously produced opaque error text ([dotnet/fsharp #19398](https://github.com/dotnet/fsharp/pull/19398)).
+
+Highlights:
+
+- **New informational warning FS3886** for a single tuple in a list literal — `[1, 2, 3]` is almost always a typo for `[1; 2; 3]`. Suppressible via `#nowarn "3886"`.
+- **New error FS3885** when `let!`/`use!` is used as the final expression in a computation expression, replacing the confusing `Unexpected symbol }`.
+- **FS0670** (the "code is not sufficiently generic" error) now suggests concrete fixes: add a type annotation, convert to a function, or mark the binding `inline`.
+- **FS3082** lists the available static parameter names (truncated at 5) when an unknown one is supplied.
+- **FS0072** ("Lookup on object of indeterminate type...") rewritten in plain English with an example annotation.
+- **Tuple-shaped errors** now say "is a tuple of type" / "given a tuple of type" when the actual type is a tuple, and the parser hints at unintended commas instead of reporting a generic missing argument.
+- **Elif chains without a final `else`** now report "missing else branch" instead of a confusing type-mismatch error.
+- **FS0597** reworked with a concrete example: `printfn "%s" (arg.Trim())`.
+- **SRTP errors on function or tuple types** now append the hint "or the operator may not be in scope".
+
+```fsharp
+let xs = [1, 2, 3]
+// warning FS3886: This list contains a single tuple element. Did you mean to use ';' instead of ','?
+
+let compute x =
+    async {
+        let! result = doWorkAsync x
+    }
+// error FS3885: 'let!' / 'use!' cannot be the final construct in a computation expression.
+```
+
+## Codegen correctness — fewer runtime crashes from valid F# code
+
+Several long-standing IL-emit bugs that produced `TypeLoadException`, `InvalidProgramException`, or stack corruption at runtime for otherwise valid F# programs are now fixed ([dotnet/fsharp #19338](https://github.com/dotnet/fsharp/pull/19338)). This continues the codegen-correctness work started in Preview 3, which dropped the F# compiler's ILVerify error count from 551 to 56.
+
+Fixes in this preview:
+
+- `voidptr` in delegate generic arguments and interface instantiations is now correctly encoded as `nativeint` in closure metadata.
+- Tail calls are now suppressed in methods that use `NativePtr.stackalloc` / `localloc`, where they were unsafe.
+- Generic type parameter constraints are no longer stripped from `Specialize<>` closure methods.
+- The `box` instruction is no longer emitted spuriously for literal upcasts (including default parameter values) or for decimal literals in debug builds.
+
+## Signature generation roundtrips cleanly
+
+`FSharpCheckFileResults.GenerateSignature` and `fsc --sig` now produce signatures that roundtrip-compile against their implementations across a much wider range of inputs ([dotnet/fsharp #19586](https://github.com/dotnet/fsharp/pull/19586), [dotnet/fsharp #19609](https://github.com/dotnet/fsharp/pull/19609)). A corpus-wide sweep of 1,483 standalone test files was used to drive out the remaining bugs.
+
+Fixes include:
+
+- Recursive module `do` bindings no longer leak compiler-generated `val`s into the signature.
+- Literal values in attribute arguments are preserved as identifiers instead of being reduced to constants.
+- Struct types now carry the correct `[<NoComparison>]` / `[<NoEquality>]` attributes in display.
+- Backtick escaping is correct for identifiers that themselves contain backticks.
+- The `private` keyword is placed correctly in prefix-style type abbreviation signatures.
+- Types without visible constructors now emit the `[<Class>]` attribute.
+- `namespace global` is detected on the `fsc --sig` path, not just through the FCS API.
+
+The on-disk pickle format is unchanged — old compilers read new DLLs identically. Display improvements also flow into IDE tooltips.
+
+## Missing implementations for C# `abstract override` are now caught at compile time
+
+When a non-abstract F# class inherited from a C# class containing `abstract override` members (for example `public abstract override string ToString()`), the F# compiler did not flag the missing implementation, and the program failed at runtime with `TypeLoadException`. The compiler now correctly reports FS0365 at compile time ([dotnet/fsharp #19503](https://github.com/dotnet/fsharp/pull/19503)).
+
+```csharp
+// C#
+public abstract class Base {
+    public abstract override string ToString();
+}
+```
+
+```fsharp
+// F#
+type Derived() =
+    inherit Base()
+// error FS0365: No implementation was given for 'Base.ToString() : string'
+```
+
+## Faster IDE responsiveness with generative type providers
+
+Solutions using generative type providers no longer pin the IDE language service at high CPU. `TypeStructure.GetHashCode` previously computed an O(n) hash over the underlying `TypeToken[]` on every cache lookup, and the type-subsumption cache rehashed all entries on each `rebuildStore`. The hash is now precomputed once at construction (O(1) lookup), with a fast-path equality check that compares cached hashes before token arrays ([dotnet/fsharp #19369](https://github.com/dotnet/fsharp/pull/19369)).
+
+The reported scenario — a multi-project solution referencing a generative type provider with ~15 provided types — saw sustained ~150% CPU on the F# language service that no longer reproduces. Thank you [@OnurGumus](https://github.com/OnurGumus) for this contribution!
+
+## Bug fixes and other improvements
+
+**Computation expressions and SRTP**
+
+- Pattern-based extension method lookups for computation-expression members (`Bind`, `Delay`, `Run`, etc.) and for the `Dispose` pattern in `use` bindings now honor accessibility and type-compatibility checks. Previously they could resolve to private extension methods or methods on incompatible types ([dotnet/fsharp #19536](https://github.com/dotnet/fsharp/pull/19536)). Thank you [@evgTSV](https://github.com/evgTSV)!
+- Fixed `seq { }` `try`/`with` running the handler body twice when the handler itself yielded ([dotnet/fsharp #19660](https://github.com/dotnet/fsharp/pull/19660)).
+- Fixed a `builder@` variable leak in resumable state machines introduced by the Preview 2 task-builder optimization ([dotnet/fsharp #19630](https://github.com/dotnet/fsharp/pull/19630)). Thank you [@majocha](https://github.com/majocha)!
+- Fixed SRTP trait call overload resolution returning the wrong typed value across multiple FSI submissions ([dotnet/fsharp #19471](https://github.com/dotnet/fsharp/pull/19471)).
+- Fixed a `NullReferenceException` when calling virtual `Object` methods on value types through inline SRTP code ([dotnet/fsharp #8098](https://github.com/dotnet/fsharp/pull/8098)).
+
+**Symbols API, tooltips, and completion**
+
+- `CLIEvent` properties are now reported as events through the Symbols API: `IsEvent` returns `true`, the XML doc signature uses the `E:` prefix, and tooltips render `event EventName` ([dotnet/fsharp #18584](https://github.com/dotnet/fsharp/pull/18584)).
+- Methods are now tagged as `Method` rather than `Member` in tooltips, fixing a regression that affected colorization and classification ([dotnet/fsharp #19507](https://github.com/dotnet/fsharp/pull/19507)).
+- Compiler-generated auto-property symbols (the `v` setter parameter and `Foo@` backing field for `member val Foo = 123 with get, set`) are no longer surfaced through `GetAllUsesOfAllSymbolsInFile` ([dotnet/fsharp #19498](https://github.com/dotnet/fsharp/pull/19498)).
+- Completion now consistently filters obsolete fields and events ([dotnet/fsharp #13512](https://github.com/dotnet/fsharp/pull/13512)).
+- `IlxGen` now emits the `CompilationMapping` attribute for generic values ([dotnet/fsharp #19643](https://github.com/dotnet/fsharp/pull/19643)). Thank you [@auduchinok](https://github.com/auduchinok)!
+
+**Compiler correctness**
+
+- `AttributeUsage.AllowMultiple` is now correctly inherited through C#-defined attribute hierarchies — the inheritance walk previously only consulted F# augmentation data and missed IL-imported supertypes ([dotnet/fsharp #19315](https://github.com/dotnet/fsharp/pull/19315)). Thank you [@edgarfgp](https://github.com/edgarfgp)!
+- Type definitions no longer emit duplicate or incorrect metadata: DU case names that match IWSAM member names no longer produce duplicate property entries, `DefaultAugmentation(false)` no longer causes duplicate method-table entries, and abstract event `add_`/`remove_` accessors are now emitted with the `SpecialName` flag ([dotnet/fsharp #19341](https://github.com/dotnet/fsharp/pull/19341)).
+- Fixed FS0452 in quotations that pattern-match an empty string (`match s with "" -> ...`) — the empty-string optimization to `mkNonNullTest` had no quotation translator handler ([dotnet/fsharp #19532](https://github.com/dotnet/fsharp/pull/19532)).
+- Fixed an incorrect range on warning 20 inside sequential expressions ([dotnet/fsharp #5735](https://github.com/dotnet/fsharp/pull/5735)).
+
+<!-- Filtered features (significant engineering work, but too niche for release notes):
+  - TypedTreeOps.fs split into 7 focused files (#19521): pure refactor, no user-visible change.
+  - EraseUnions DU-based domain model refactor (#19518): internal refactor, no user-visible change.
+  - Compiler options parser tests (#13878 / #19554, techiedesu): test-only.
+  - In-proc fsi exit shadowing in tests (#19407, majocha): test infra.
+  - Flaky CI fixes for cancellation/async/task tests (#19569, #19632): test infra.
+  - IL baseline duality elimination (#19611), .bsl baseline refresh (#19603), .sln → .slnx (#19585), Release CI legs (#19642): build/test infra.
+  - Heap dumps for crash diagnostics (#19607): internal diagnostics.
+  - SDK bump to 10.0.202 (#19606), build.sh quoting (#19570), devcontainer workaround removal (#19549), VS image / VSMinorVersion bumps (#19584, #19590, #19610, #19639, #19650, #19651, #19652), PAT → WIF migration (#19598), source-build-assets repo (#19573), OneLocBuild check-in (#19418): repo/build infra.
+  - All `Add regression test: ...` PRs (#19463–#19497, #19528–#19535, #19579–#19581, #19613): tests-only, generated by Regression PR Shepherd.
+  - Agentic workflow / repo-assist / aw-auto-update PRs (#19495, #19499, #19500, #19510, #19516, #19519, #19520, #19526, #19556, #19558, #19567, #19624, #19655): repo automation.
+  - All dotnet-maestro and source-code update PRs: dependency churn.
+  - maxBy/minBy first-element documentation/tests (#19464): docs/test only.
+-->
+
+## Community contributors
+
+Thank you contributors! ❤️
+
+- [@auduchinok](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aauduchinok)
+- [@edgarfgp](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aedgarfgp)
+- [@evgTSV](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3AevgTSV)
+- [@majocha](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Amajocha)
+- [@Martin521](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3AMartin521)
+- [@omajid](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Aomajid)
+- [@OnurGumus](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3AOnurGumus)
+- [@techiedesu](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3Atechiedesu)
+- [@Youssef1313](https://github.com/dotnet/fsharp/pulls?q=is%3Apr+is%3Amerged+author%3AYoussef1313)
+
+F# updates:
+
+- [F# release notes](https://fsharp.github.io/fsharp-compiler-docs/release-notes/About.html)
+- [dotnet/fsharp repository](https://github.com/dotnet/fsharp)