Reenable β-reduction of immediately-invoked F#-defined generic delegates

[brianrourkeboll]

Description

• Reenable β-reduction and subsequent reoptimization of immediately-invoked F#-defined generic delegates.

  This optimization was added as part of https://github.com/fsharp/fslang-design/blob/main/tooling/FST-1034-lambda-optimizations.md in [RFC FS-1087, FS-1097, FS-1098] tasks, resumable state machines, inline on parameters #6811. It originally applied to both generic and non-generic delegates.

  I made use of this optimization when writing https://github.com/fsprojects/FSharp.Collections.Builders — but it turns out that the optimization has no longer been applied to code like that since .NET 7.

  Fix immediate delegate invoke #12920 (1b39cde) effectively restricted the optimization to non-generic delegates here:

  let (|DelegateInvokeExpr|_|) g expr =
      match expr with
  -   | Expr.App ((Expr.Val (invokeRef, _, _)) as iref, fty, tyargs, (f :: args), m) 
  -       when invokeRef.LogicalName = "Invoke" && isFSharpDelegateTy g (tyOfExpr g f) -> 
  -           Some(iref, fty, tyargs, f, args, m)
  +   | Expr.App ((Expr.Val (invokeRef, _, _)) as delInvokeRef, delInvokeTy, [], [delExpr;delInvokeArg], m) 
  +       when invokeRef.LogicalName = "Invoke" && isFSharpDelegateTy g (tyOfExpr g delExpr) -> 
  +           Some(delInvokeRef, delInvokeTy, delExpr, delInvokeArg, m)
      | _ -> None

  I.e., before, (|DelegateInvokeExpr|_|) would match delegates with type arguments; after, it would not.

  It does not seem like that change was essential to the fix for Expected type error on delegate #5607. I added the tests from Fix immediate delegate invoke #12920 as emitted IL tests here, and they:

  • Compile (which was the problem in Expected type error on delegate #5607)
  • Show that the optimization is applied to both non-generic and generic delegates.

Checklist

• Test cases added.
• Release notes entry updated.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/9.0.300.md