Fix #17904 and #19020

[edgarfgp]

Fixes #17904 and #19020.

Problem

Two related bugs share the same root cause — [<return: X>] prefix attributes on a binding aren't routed to the return-value metadata slot:

• [<return: X>] not respected on class members #19020: [<return: X>] on class members is silently dropped from IL. The attribute appears in source, but reflection on the method's return parameter finds nothing.
• F# compiler doesn't respect different attribute targets when checking for Multiple appliance #17904: [<X>] and [<return: X>] on the same member are incorrectly flagged as duplicates under AllowMultiple = false, because both end up in Val.Attribs together rather than being separated by metadata target.

Before

open System

[<AttributeUsage(AttributeTargets.ReturnValue)>]
type DescAttribute() = inherit Attribute()

type T() =
    [<return: Desc>]
    member _.Foo a = a + 1

// Reflection finds 0 attributes — silently dropped (#19020)
typeof<T>.GetMethod("Foo").ReturnParameter.GetCustomAttributes(typeof<DescAttribute>, false)

[<AttributeUsage(AttributeTargets.All)>]
type AttrAttribute() = inherit Attribute()

type T() =
    [<Attr>]
    [<return: Attr>]                      // error FS0429: AllowMultiple=false (#17904)
    member _.Foo() = ()

After

// Reflection finds 1 attribute on the return parameter
typeof<T>.GetMethod("Foo").ReturnParameter.GetCustomAttributes(typeof<DescAttribute>, false).Length = 1

// [<Attr>] (method) and [<return: Attr>] (return value) target different
// metadata slots — no longer a duplicate.
type T() =
    [<Attr>]
    [<return: Attr>]
    member _.Foo() = ()                   // compiles cleanly

Genuine duplicates still error:

type T() =
    [<return: Attr>]
    [<return: Attr>]                      // error FS0429: still flagged
    member _.Foo() = ()

type T() =
    [<Attr>]
    [<method: Attr>]                      // error FS0429: still flagged (same target)
    member _.Foo() = ()

Solution

Move the rotation to the parser stage. In mkSynBinding (SyntaxTreeOps.fs), any SynAttribute whose Target = Some "return" is moved out of the binding's prefix attribute list into SynValData.SynValInfo.retInfo. From that point onward, every downstream consumer (type-checker, IL emit, FCS Symbols API) sees the attribute in the semantically correct slot, regardless of where the user wrote it.

The discriminator is syntactic — synAttr.Target = Some "return" — not the typed constrainedTargets bitmap. Attributes without a return: prefix are never touched, so [<CompilationRepresentation(...)>] on Option.Value is unaffected and FSharp.Core builds cleanly.

[github-actions]

❗ Release notes required

---

✅ 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

[edgarfgp]

@T-Gro I think the CI is stuck or it has not even started.

[Copilot]

Pull request overview

This PR fixes two related attribute-handling bugs by ensuring [<return: ...>] prefix attributes on bindings are routed to the return-value metadata slot early (at syntax construction time), so downstream phases (type-checking, IL emit, FCS symbols) see them in the correct place.

Changes:

• Rotate [<return: ...>] prefix attributes from binding attributes into SynValInfo.retInfo during mkSynBinding.
• Update downstream consumers (notably active pattern return-kind detection and binding attribute type-checking) to read return attributes from return-info rather than Val.Attribs.
• Add component regression tests for issues #17904 and #19020, plus a release-notes entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/FSharp.Compiler.ComponentTests/Language/AttributeCheckingTests.fs	Adds regression tests covering return-attribute emission and duplicate detection across targets.
src/Compiler/SyntaxTree/SyntaxTreeOps.fs	Implements parser-stage rotation of [<return: ...>] attributes into SynValInfo.retInfo.
src/Compiler/Checking/NameResolution.fs	Adjusts active-pattern return-kind logic to look for return attributes in ValReprInfo return info.
src/Compiler/Checking/Expressions/CheckExpressions.fs	Removes typechecker-stage rotation and re-plumbs return attribute collection/typechecking.
docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	Documents the bug fixes in release notes.

[github-actions]

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: changes attribute routing in IL emit for return-value attributes

Generated by PR Tooling Safety Check · ● 2.1M · ◷

[auduchinok]

@edgarfgp Could you please add FCS tests accessing the attributes via both the member itself and the return value?

[edgarfgp]

@edgarfgp Could you please add FCS tests accessing the attributes via both the member itself and the return value?

@auduchinok Done 72f0e53