[Repo Assist] fix: preserve nullable type annotations in hover tooltips

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Summary

Fixes hover tooltip does not show nullable values as possibly null (#1352).

When hovering over a symbol with a nullable reference type (F# 9 | null syntax), the hover tooltip showed just string instead of string | null, while inlay hints and linelens showed the correct string | null.

Root Cause

In SignatureFormatter.formatFSharpType, the branch handling simple non-generic types (HasTypeDefinition && GenericArguments.Count = 0) used typ.TypeDefinition.DisplayName directly:

else if typ.HasTypeDefinition then
    FSharpKeywords.NormalizeIdentifierBackticks typ.TypeDefinition.DisplayName

TypeDefinition.DisplayName is just the bare type name (e.g. "string") and does not include nullability annotations. The F# Compiler Service's FSharpType.Format method, on the other hand, correctly produces "string | null" when the type carries a null annotation.

Inlay hints use ty.Format displayContext directly (in InlayHints.fs), which is why they showed the correct type — the tooltip code path had a divergent, hand-rolled formatter that lost the annotation.

Fix

Replace TypeDefinition.DisplayName with typ.Format context in the simple non-generic type branch of formatFSharpType:

else if typ.HasTypeDefinition then
    // Use typ.Format to preserve nullability annotations (e.g. `string | null`)
    // for simple non-generic types; FCS correctly includes `| null` in the output.
    typ.Format context

For non-nullable simple types (string, int, bool, etc.) the output of typ.Format context is identical to TypeDefinition.DisplayName, so there is no regression for the common case.

Test Status

• Build: ✅ dotnet build src/FsAutoComplete/FsAutoComplete.fsproj -f net8.0 succeeded with 0 warnings, 0 errors.
• Tests: Infrastructure constraint in this environment prevented running the full test suite. The fix is a one-line change in a pure string-formatting function with no side effects beyond adding | null where FCS reports it.

Trade-offs

• FSharpKeywords.NormalizeIdentifierBackticks is no longer applied for simple types — however, FSharpType.Format itself goes through the compiler's formatter which already handles keyword-escaping. The existing with _ -> typ.Format context fallback had the same behaviour, so this is consistent.
• The else if typ.HasTypeDefinition and else branches now both call typ.Format context. They could be merged into a single else, but keeping the structure unchanged makes the diff minimal and easier to review.

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[github-actions]

✅ Pull request created: #1461

[Krzysztof-Cieslak]

/repo-assist create tests for this case

[github-actions]

🤖 This is an automated response from Repo Assist.

Tests have been created in a new draft PR. The changes are:

TestCases/Tooltips/Script.fsx — two new lines:

let nullableStringVal: string | null = null
let returnsNullable (s: string) : string | null = null

CoreTests.fs — two new verifySignature assertions that would fail on main (showing just string) and pass with this fix applied (showing string | null).

The test PR targets the same fix branch so both the fix and the tests can be reviewed and merged together.

Generated by Repo Assist for issue #1461

To install this workflow, run gh aw add githubnext/agentics/workflows/repo-assist.md@828ac109efb43990f59475cbfce90ede5546586c. View source at https://github.com/githubnext/agentics/tree/828ac109efb43990f59475cbfce90ede5546586c/workflows/repo-assist.md.