Suppress hover/symbol resolution for wildcard _ patterns inside member _.…

[Copilot]

When a member uses _ as its instance identifier, tooling was incorrectly treating wildcard _ patterns in the member body as references to that instance. This showed misleading hover text like val _: T for discard patterns that should resolve to nothing.

• Compiler service resolution

  • Teach GetDeclItemsForNamesAtPosition to stop fallback identifier lookup for bare _ when precise name resolution did not produce a result.
  • This keeps wildcard patterns from being resolved through the member's synthetic _ instance identifier while preserving normal member-instance behavior elsewhere.

• Tooltip regressions

  • Add focused service tests covering discard patterns inside member _.… bodies:
    • lambda parameter discard
    • let binding discard
  • Assert that these locations produce an empty tooltip instead of val _: T.

• Behavioral example

  type T () =
      member _.M () =
          fun _ -> ()
          // no hover
  
      member _.N () =
          let _ = () in ()
          // no hover

[T-Gro]

Looks good overall — the fix is correct and well-scoped.

The placement of the isDiscardIdentifier guard after the NameResResult.Members(FilterRelevantItems ...) branch is exactly right: it only short-circuits the fallback lookup when precise name resolution didn't produce results, preventing the identifier _ in scope (i.e., the member's self-identifier) from being incorrectly surfaced as tooltip text for wildcard/discard patterns.

Key observations:

• The condition (None, Some ["_"]) correctly limits this to bare _ — it won't interfere with dotted access like _.Property (which would be Some ["_"; "Property"]).
• Since residueOpt = None is specific to the tooltip/symbol-lookup callers (not the completion path which passes Some partialName.PartialIdent), completion behavior is unaffected.
• The fix also benefits GetSymbolUsesAtLocation and GetF1Keyword, which share the same call path — a nice consistency win.

Minor suggestions (non-blocking):

1. A short inline comment above the guard (e.g., // Wildcard _ should not resolve to the member's synthetic self-identifier via fallback) would help future readers understand why this case is separated from the general fallback.

2. Consider adding a test for match x with _ -> ... inside a member _.M() body — this is another common discard position where the old behavior would have shown misleading hover text.

3. Consider a negative test confirming that a named self-identifier (e.g., member this.M() = ...; hover over 'this') is NOT affected by the fix — purely for regression confidence.

[github-actions]

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