Function application signature help: only first parameter is ever highlighted

[cartermp]

Consider this F# script:

let f a b = ()

f 1

Now hit space after the 1 and see that the first parameter is highlighted:

The second parameter in the tooltip needs to be highlighted, since that is the tooltip that we're "on".

[baronfel]

#757 doesn't fix this one, the underlying FSharpMethodGroup that we get from FCS is only reporting 1 curried parameter group in this instance so the textDocument/signatureHelp response for this one is

{
    "signatures": [
        {
            "label": "val f : a:'a -> b:'b -> unit",
            "documentation": {
                "kind": "markdown",
                "value": ""
            },
            "parameters": [
                {
                    "label": "a",
                    "documentation": "'a"
                }
            ]
        }
    ],
    "activeParameter": 1
}

note the single parameter here. If we were reporting all parameters, then this would 'just work'

[github-actions]

🤖 This is an automated response from Repo Assist.

@baronfel's comment from 2021 identified the root cause well: GetMethods (the FCS API used here) only exposes the first curried parameter group of a curried F# function. For let f a b = (), it returns a single MethodGroupItem whose Parameters array contains only a, not b. So activeParameter = 1 points past the end of the parameters array, and the editor doesn't highlight anything (or highlights the last known one).

The fix would require using mfv.CurriedParameterGroups directly (we already have mfv in scope in getSignatureHelpForFunctionApplication) instead of relying on GetMethods. This would mean constructing parameter info from the curried groups and exposing it through SignatureHelpInfo. It's a moderate refactor of SignatureHelp.fs and the LSP server, but very doable.

If someone wants to take a stab at this, the relevant code is in:

• src/FsAutoComplete.Core/SignatureHelp.fs — getSignatureHelpForFunctionApplication (around line 82, where mfv.CurriedParameterGroups is already accessible)
• src/FsAutoComplete/LspServers/AdaptiveFSharpLspServer.fs — TextDocumentSignatureHelp handler (lines ~1009–1028)

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