Add fsharp.md for .NET 11 Preview 2 (#10275)

* Add fsharp.md for .NET 11 Preview 2

* Update F# release notes for .NET 11 Preview 2

This release includes new language features, performance improvements, and bug fixes for F# in .NET 11 Preview 2. Highlights include simplified DIM interface hierarchies, overload resolution caching, Big-O complexity documentation, and enhancements to query expressions and IDE features.

---------

Co-authored-by: Tomas Grosup <tomasgrosup@microsoft.com>

Author: jamesmontemagno

release-notes/11.0/preview/preview2/fsharp.md

@@ -0,0 +1,110 @@
+# F# in .NET 11 Preview 2 - Release Notes
+
+Here's a summary of what's new in F# in this Preview 2 release:
+
+- [New language feature: Simplified DIM interface hierarchies (preview)](#simplified-dim-interface-hierarchies)
+- [Compiler performance: Overload resolution caching (preview)](#overload-resolution-caching)
+- [FSharp.Core: Big-O complexity documentation for all collection functions](#big-o-complexity-documentation)
+- [Query expression fixes for EF Core and LINQ providers](#query-expression-fixes)
+- [IDE: Find All References and Rename overhaul](#find-all-references-and-rename-overhaul)
+- [Bug fixes](#bug-fixes-and-other-improvements)
+
+## Simplified DIM interface hierarchies
+
+When a C# interface provides a Default Interface Member (DIM) implementation for a base interface slot, F# no longer requires you to explicitly implement the already-covered slot. Given these C# interfaces:
+
+```csharp
+public interface IA { int M(); }
+public interface IB : IA {
+    new int M();
+    int IA.M() => this.M() + 100;  // DIM covers IA.M
+}
+```
+
+F# previously required implementing both `IA` and `IB`. Now you only implement `IB`:
+
+```fsharp
+type C() =
+    interface IB with member _.M() = 42
+
+(C() :> IB).M()   // 42
+(C() :> IA).M()   // 142 (DIM forwards: this.M() + 100)
+```
+
+This also works with diamond inheritance, generic interfaces, properties, events, structs, and object expressions. Requires `--langversion:preview`. ([RFC FS-1336](https://github.com/fsharp/fslang-design/pull/826), [Language suggestion #1430](https://github.com/fsharp/fslang-suggestions/issues/1430), [PR #19241](https://github.com/dotnet/fsharp/pull/19241))
+
+## Overload resolution caching
+
+A new preview feature (`--langversion:preview`) caches overload resolution results for repeated method calls with the same argument types. In a benchmark compiling a file with 5,000 `Assert.Equal(x, y)` calls (a common pattern in test projects):
+
+| Metric | .NET 9 SDK | .NET 10 SDK | This feature |
+|---|---|---|---|
+| Typecheck time | 40.8s | 6.5s | **2.9s** |
+| GC0 collections | — | 115 | **21** |
+
+The cache is keyed on method group identity, argument types, and return type. It is automatically disabled for named arguments, SRTP constraints, and other context-dependent scenarios. ([#18807](https://github.com/dotnet/fsharp/issues/18807), [PR #19072](https://github.com/dotnet/fsharp/pull/19072))
+
+## Big-O complexity documentation
+
+All 462 functions across `Array`, `List`, `Seq`, `Map`, and `Set` now include Big-O complexity in their XML docs, visible in IDE tooltips:
+
+```fsharp
+/// <remarks>This is an O(n) operation, where n is the length of the first list.</remarks>
+val append: list1: 'T list -> list2: 'T list -> 'T list
+
+/// <remarks>This is an O(log(n)) operation.</remarks>
+val tryFind: key: 'Key -> table: Map<'Key, 'Value> -> 'Value option
+```
+
+([PR #19240](https://github.com/dotnet/fsharp/pull/19240))
+
+## Query expression fixes
+
+This release fixes **10 longstanding bugs** affecting `query { }` expressions and LINQ provider compatibility, some open for over 10 years. This significantly improves the F# story for Entity Framework Core, Cosmos DB, and other LINQ providers:
+
+- Tuple joins and groupBy now work correctly — `join b on ((a.Id1, a.Id2) = (b.Id1, b.Id2))` finally works. ([#7885](https://github.com/dotnet/fsharp/issues/7885), [#47](https://github.com/dotnet/fsharp/issues/47))
+- Tuple projections preserve `IQueryable` instead of falling back to `Enumerable.Select`, enabling `ToListAsync()` and other async EF Core operations. ([#3782](https://github.com/dotnet/fsharp/issues/3782), [#15133](https://github.com/dotnet/fsharp/issues/15133))
+- Array indexing in LINQ expressions generates proper `ArrayIndex` nodes, enabling Cosmos DB to translate array access. ([#16918](https://github.com/dotnet/fsharp/issues/16918))
+- Anonymous record field ordering in expression trees is now consistent regardless of declaration order. ([#11131](https://github.com/dotnet/fsharp/issues/11131))
+- Conditional queries without an else branch no longer cause type mismatch errors. ([#3445](https://github.com/dotnet/fsharp/issues/3445))
+- Fixed false FS1182 warnings (unused variable) for query variables in `where`, `let`, `join`, and `select` clauses. ([#422](https://github.com/dotnet/fsharp/issues/422))
+
+([PR #19243](https://github.com/dotnet/fsharp/pull/19243))
+
+## Find All References and Rename overhaul
+
+A comprehensive pass over the Find All References (FAR) and Rename Symbol features fixes **15+ longstanding bugs**, some open for over 7 years:
+
+- Active pattern cases in signature files now found correctly. ([#19173](https://github.com/dotnet/fsharp/issues/19173), [#14969](https://github.com/dotnet/fsharp/issues/14969))
+- Rename no longer incorrectly renames `get`/`set` keywords on properties. ([#18270](https://github.com/dotnet/fsharp/issues/18270))
+- Rename now handles operators containing `.` (e.g., `-.-`). ([#17221](https://github.com/dotnet/fsharp/issues/17221))
+- C# extension methods now found across all usages. ([#16993](https://github.com/dotnet/fsharp/issues/16993))
+- DU case tester properties (`.IsCase`) now included. ([#16621](https://github.com/dotnet/fsharp/issues/16621))
+- FAR on record types now includes copy-and-update expressions. ([#15290](https://github.com/dotnet/fsharp/issues/15290))
+- Constructor definitions now find all usages. ([#14902](https://github.com/dotnet/fsharp/issues/14902))
+- External DLL symbol searches now scoped to referencing projects only (performance). ([#10227](https://github.com/dotnet/fsharp/issues/10227))
+- `#line` directive remapping now applied correctly. ([#9928](https://github.com/dotnet/fsharp/issues/9928))
+- DU types inside modules now discoverable. ([#5545](https://github.com/dotnet/fsharp/issues/5545))
+- Synthetic event handler values no longer appear in results. ([#4136](https://github.com/dotnet/fsharp/issues/4136))
+- FAR no longer crashes on projects containing non-F# files like `.cshtml`. ([#16394](https://github.com/dotnet/fsharp/issues/16394))
+
+([PR #19252](https://github.com/dotnet/fsharp/pull/19252), [PR #19311](https://github.com/dotnet/fsharp/pull/19311))
+
+## Bug fixes and other improvements
+
+- Nullness: `int<meter>.ToString()` no longer returns `string | null` for value types. ([#17539](https://github.com/dotnet/fsharp/issues/17539))
+- Nullness: Pipe operator warnings now point at the nullable argument, not the pipe operator. ([#18013](https://github.com/dotnet/fsharp/issues/18013))
+- Nullness: Fixed false positive warning when passing non-null `AllowNullLiteral` constructor result. ([#18021](https://github.com/dotnet/fsharp/issues/18021))
+- Nullness: `not null` constraint on type extensions is now allowed. ([#18334](https://github.com/dotnet/fsharp/issues/18334))
+- Nullness: Tuple null elimination no longer over-infers non-null in pattern matching. ([#19042](https://github.com/dotnet/fsharp/issues/19042))
+- Fixed FS0229 error when reading metadata from assemblies compiled with `LangVersion < 9.0`. ([PR #19260](https://github.com/dotnet/fsharp/pull/19260))
+- Fixed FS3356 false positive for instance extension members with the same name on different types. ([PR #19260](https://github.com/dotnet/fsharp/pull/19260))
+- Fixed graph-based type checking dependency resolution for duplicate module names across files. ([PR #19280](https://github.com/dotnet/fsharp/pull/19280))
+- Fixed F# script default reference paths when an SDK directory is specified. ([PR #19270](https://github.com/dotnet/fsharp/pull/19270))
+
+...and many others.
+
+F# updates:
+
+- [F# release notes](https://fsharp.github.io/fsharp-compiler-docs/release-notes/About.html)
+- [dotnet/fsharp repository](https://github.com/dotnet/fsharp)