Fix FieldAccessException when optimizer relocates protected base-field access (#19963)

[T-Gro]

Fixes#19963.

Problem

Reading a protected (family) base-class field from a derived F# member compiles fine, but the optimizer can relocate that field load — by inlining or method/lambda splitting — into a method outside the field's family. The relocated ldfld/stfld is then illegal IL and throws System.FieldAccessException at runtime. This happens under --optimize+, the default for Release builds, regardless of --realsig.

// C# librarypublicclassBase{protectedstringField="hello";}

typeInherited()=inherit Base()memberx.Run()= x.Field // legal; compiles today[<EntryPoint>]letmain _ =if(Inherited()).Run()="hello"then0else1

--optimize- runs fine; --optimize+ inlines Run into main (which is not in Base's family) and throws:

System.FieldAccessException: Attempt by method 'Program.main(...)' to access field 'Base.Field' failed.

Cause

Protected method/base calls are already pinned in-family: they set FreeVars.UsesMethodLocalConstructs (via TOp.ILCall isProtected), which the optimizer's relocation guards honour. Protected field loads were invisible to that machinery, so the optimizer freely relocated them. Contrast under --optimize+:

memberx.Run()= x.Method()// protected method -> OKmemberx.Run()= x.Field // protected field -> FieldAccessException

Fix

Add a FreeVars.ContainsILFieldAccess flag, set for any IL field instruction (TOp.ILAsmldfld/ldflda/stfld/ldsfld/ldsflda/stsfld), and OR it into the five optimizer relocation guards so such code is not inlined or split out of its declaring family.

The flag is deliberately not read by the FS0405 escape check, so it can never reject otherwise-valid code (e.g. capturing a public field like IntPtr.Zero in a closure). Field accessibility is not available where free variables are summarised, so the flag is conservative over all IL field access; F#-defined fields use TOp.ValFieldGet and are unaffected.

Validation

• New regression tests for instance and static protected base fields under --optimize+ (red before, green after).
• Full EmittedIL suite: 1307 tests, zero baseline churn.
• Accessibility and object-expression suites unaffected.

This is also a prerequisite for enabling protected-member access from closures (#5302): the same relocation otherwise reintroduces the runtime failure there.

[github-actions]

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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/11.0.100.md

[T-Gro]

AI code review (expert-reviewer). The committed design fixes the crash but has an over-broad relocation guard that will block inlining of public IL field accesses - a regression that manifests as FS1118 when bootstrapping FSharp.Core.