Fix IsByRefLikeAttribute types being incorrectly suppressed in completion lists (#18784)

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -16,6 +16,7 @@
 * Completion: fix previous namespace considered opened [PR #18609](https://github.com/dotnet/fsharp/pull/18609)
 * Fix active pattern typechecking regression. ([Issue #18638](https://github.com/dotnet/fsharp/issues/18638), [PR #18642](https://github.com/dotnet/fsharp/pull/18642))
 * Fix nullness warnings when casting non-nullable values to `IEquatable<T>` to match C# behavior. ([Issue #18759](https://github.com/dotnet/fsharp/issues/18759))
+* Fix IsByRefLikeAttribute types being incorrectly suppressed in completion lists. Types like `Span<T>` and `ReadOnlySpan<T>` now appear correctly in IntelliSense.
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

src/Compiler/Checking/AttributeChecking.fs

@@ -441,7 +441,17 @@ let private CheckProvidedAttributes (g: TcGlobals) m (provAttribs: Tainted<IProv
 /// Indicate if a list of IL attributes contains 'ObsoleteAttribute'. Used to suppress the item in intellisense.
 let CheckILAttributesForUnseen (g: TcGlobals) cattrs _m = 
     let (AttribInfo(tref, _)) = g.attrib_SystemObsolete
-    Option.isSome (TryDecodeILAttribute tref cattrs)
+    let hasObsolete = Option.isSome (TryDecodeILAttribute tref cattrs)
+    if hasObsolete then
+        // Exclude types marked with IsByRefLikeAttribute from being considered obsolete,
+        // even if ObsoleteAttribute is present. This avoids improper suppression of types 
+        // like Span and ReadOnlySpan in completion lists due to their dual attributes.
+        match g.attrib_IsByRefLikeAttribute_opt with
+        | Some (AttribInfo(isByRefLikeTref, _)) ->
+            not (Option.isSome (TryDecodeILAttribute isByRefLikeTref cattrs))
+        | None -> true
+    else
+        false
 
 /// Checks the attributes for CompilerMessageAttribute, which has an IsHidden argument that allows
 /// items to be suppressed from intellisense.
@@ -460,8 +470,13 @@ let CheckFSharpAttributesForHidden g attribs =
      | _ -> false)
 
 /// Indicate if a list of F# attributes contains 'ObsoleteAttribute'. Used to suppress the item in intellisense.
-let CheckFSharpAttributesForObsolete g attribs = 
-    not (isNil attribs) && (HasFSharpAttribute g g.attrib_SystemObsolete attribs)
+let CheckFSharpAttributesForObsolete (g:TcGlobals) attribs = 
+    not (isNil attribs) && 
+    (HasFSharpAttribute g g.attrib_SystemObsolete attribs) &&
+    // Exclude types marked with IsByRefLikeAttribute from being considered obsolete,
+    // even if ObsoleteAttribute is present. This avoids improper suppression of types 
+    // like Span and ReadOnlySpan in completion lists due to their dual attributes.
+    not (HasFSharpAttributeOpt g g.attrib_IsByRefLikeAttribute_opt attribs)
 
 /// Indicate if a list of F# attributes contains 'ObsoleteAttribute'. Used to suppress the item in intellisense.
 /// Also check the attributes for CompilerMessageAttribute, which has an IsHidden argument that allows

src/Compiler/TypedTree/TcGlobals.fs

@@ -1481,6 +1481,7 @@ type TcGlobals(
   member val enum_DynamicallyAccessedMemberTypes = findOrEmbedSysPublicType "System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes"
 
   member val attrib_SystemObsolete = findSysAttrib "System.ObsoleteAttribute"
+  member val attrib_IsByRefLikeAttribute_opt = tryFindSysAttrib "System.Runtime.CompilerServices.IsByRefLikeAttribute"
   member val attrib_DllImportAttribute = tryFindSysAttrib "System.Runtime.InteropServices.DllImportAttribute"
   member val attrib_StructLayoutAttribute = findSysAttrib "System.Runtime.InteropServices.StructLayoutAttribute"
   member val attrib_TypeForwardedToAttribute = findSysAttrib "System.Runtime.CompilerServices.TypeForwardedToAttribute"

src/Compiler/TypedTree/TcGlobals.fsi

@@ -498,6 +498,8 @@ type internal TcGlobals =
 
     member attrib_SystemObsolete: BuiltinAttribInfo
 
+    member attrib_IsByRefLikeAttribute_opt: BuiltinAttribInfo option
+
     member attrib_ThreadStaticAttribute: BuiltinAttribInfo option
 
     member attrib_TypeForwardedToAttribute: BuiltinAttribInfo

tests/FSharp.Compiler.Service.Tests/CompletionTests.fs

@@ -360,3 +360,14 @@ module Module =
 
 """
     assertHasNoItemsWithNames ["E"] info
+
+#if NETCOREAPP
+[<Fact>]
+let ``Span appears in completion and is not marked obsolete`` () =
+    let info = Checker.getCompletionInfo """
+let test = System.Sp{caret}
+"""
+    // Verify that Span appears in completion when typing "System.Sp"
+    // and is not suppressed due to IsByRefLikeAttribute
+    assertHasItemWithNames ["Span"] info
+#endif