Align inlay hints with trailing tuple arguments by jakebailey · Pull Request #64410 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-09-23T18:01:54Z

It was easier to just write what I meant on #64363 than to describe it better.

Trailing fixed tuple elements correspond to the last arguments, not the first positions after a rest element. Without aligning from the end, inlay hints point to the wrong parameter and can hide names of later parameters.

Copilot

Copilot review overview

🔵 Needs a closer look

Trailing fixed elements are still mapped incorrectly when tuples contain multiple generic variadic elements.

Review effort: Balanced
Findings: None

What changed in this PR

Aligns language-service inlay hints with trailing tuple arguments and preserves hints after unnamed parameters.

Changes:

Matches trailing tuple elements from the end.
Shares tuple-element counting helpers.
Adds regression tests and baseline expectations.

File	Description
`tsc/testdata/baselines/reference/fourslash/inlayHints/inlayHintsTupleRestTrailing.baseline`	Records expected inlay hints.
`tsc/internal/ls/signaturehelp.go`	Shares required tuple-element counting.
`tsc/internal/ls/inlay_hints.go`	Adjusts hint mapping; multiple generic variadic elements remain unresolved.
`tsc/internal/fourslash/tests/inlayHintsTupleRestTrailing_test.go`	Adds regression coverage.
`tsc/internal/checker/exports.go`	Exposes trailing tuple-element counting.

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Jake Bailey (jakebailey) · 2026-09-23T18:18:43Z

Needs a closer look

Trailing fixed elements are still mapped incorrectly when tuples contain multiple generic variadic elements.

This is a review comment in disguise 😄

A spread can leave the effective argument count unknown even when explicit arguments follow it. Those trailing arguments still have known positions from the end of the call.

Optional tuple elements may be omitted, so their names cannot always be inferred from the end of a call. An argument before a known trailing suffix must not be labeled as that suffix either.

Copilot

Copilot review overview

🔵 Needs a closer look

Tuple spreads and omitted optional suffixes can still produce incorrect parameter hints.

Review effort: Balanced
Findings: None

Optional and variable tuple elements make the next parameter position uncertain. Keep hints for positions known from the end of a required tuple suffix, but omit names that could refer to a different argument.

Jake Bailey (jakebailey) · 2026-09-23T22:24:45Z

Leave it to copilot review to keep finding weird edge cases, sigh

Copilot

Copilot review overview

🔵 Needs a closer look

Multiple moderate inlay-hint mapping regressions remain unresolved.

Review effort: Balanced
Findings: None

Previously missed (1)

In code that hasn't changed since last review

Avoid aligning optional trailing tuple elements as required

tsc/internal/ls/inlay_hints.go:897

This alignment uses trailingCount, so an optional suffix is treated as present whenever the call has enough arguments. For declare function f(...args: [...head: number[], tail?: string]); f(1), index == 0 is remapped to tail, even though the optional tail may be omitted and 1 belongs to head; this regresses the existing start-to-end mapping for optional trailing tuples. Only apply this known-arity remapping when all trailing fixed elements are required (and add a regression case for an omitted optional suffix).

Daniel Rosenwasser (DanielRosenwasser) · 2026-09-25T00:22:21Z

-			associatedName := associatedNames[index]
+		restArgumentCount := argumentCount - paramCount
+		firstVariableIndex := tupleType.FixedLength()
+		trailingCount := checker.GetEndElementCount(tupleType, checker.ElementFlagsFixed)


Maybe trailingFixedCount

Daniel Rosenwasser (DanielRosenwasser) · 2026-09-25T00:23:10Z

+				index = len(elementInfos) - offsetFromEnd
+			case offsetFromEnd > trailingCount:
+				return nil
+			case argumentCount >= 0 && restArgumentCount >= firstVariableIndex+trailingCount:


I have mostly been able to track this stuff, but I'm starting to find it hard to keep track of in this branch. Maybe you could add some comments at each branch to show concrete situations that we're trying to handle?

Jake Bailey (jakebailey) added 2 commits September 22, 2026 19:46

Test inlay hints with trailing tuple rest elements

a079981

Align inlay hints with trailing tuple arguments

755ff35

Trailing fixed tuple elements correspond to the last arguments, not the first positions after a rest element. Without aligning from the end, inlay hints point to the wrong parameter and can hide names of later parameters.

Jake Bailey (jakebailey) requested review from Andrew Branch (andrewbranch) and Gabriela Araujo Britto (gabritto) and a balanced review from Copilot September 23, 2026 18:01

typescript-automation Bot assigned Jake Bailey (jakebailey) Sep 23, 2026

github-project-automation Bot added this to PR Backlog Sep 23, 2026

typescript-automation Bot added Author: Team For Backlog Bug PRs that fix a backlog bug labels Sep 23, 2026

github-project-automation Bot moved this to Not started in PR Backlog Sep 23, 2026

Copilot started reviewing on behalf of Jake Bailey (jakebailey) September 23, 2026 18:02 View session

Jake Bailey (jakebailey) mentioned this pull request Sep 23, 2026

Fix inlay hints for trailing required rest elements #64363

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Gabriela Araujo Britto (gabritto) approved these changes Sep 23, 2026

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github-project-automation Bot moved this from Not started to Needs merge in PR Backlog Sep 23, 2026

Jake Bailey (jakebailey) added 4 commits September 23, 2026 11:42

Test inlay hints with multiple variadic tuple elements

96b101a

Align trailing tuple hints after generic spreads

edf2f7b

A spread can leave the effective argument count unknown even when explicit arguments follow it. Those trailing arguments still have known positions from the end of the call.

Test trailing tuple hints after spreads

6bc1039

Avoid incorrect trailing tuple hints after spreads

02b185f

Optional tuple elements may be omitted, so their names cannot always be inferred from the end of a call. An argument before a known trailing suffix must not be labeled as that suffix either.

Jake Bailey (jakebailey) requested review from Gabriela Araujo Britto (gabritto) and a balanced review from Copilot September 23, 2026 21:55

Copilot started reviewing on behalf of Jake Bailey (jakebailey) September 23, 2026 21:56 View session

Copilot AI reviewed Sep 23, 2026

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Jake Bailey (jakebailey) added 2 commits September 23, 2026 15:12

Test ambiguous tuple spread parameter hints

3f6f1b8

Omit uncertain hints after tuple spreads

c6ca73c

Optional and variable tuple elements make the next parameter position uncertain. Keep hints for positions known from the end of a required tuple suffix, but omit names that could refer to a different argument.

Jake Bailey (jakebailey) requested a balanced review from Copilot September 23, 2026 22:24

Copilot started reviewing on behalf of Jake Bailey (jakebailey) September 23, 2026 22:24 View session

Copilot AI reviewed Sep 23, 2026

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Daniel Rosenwasser (DanielRosenwasser) reviewed Sep 25, 2026

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Align inlay hints with trailing tuple arguments - #64410

Align inlay hints with trailing tuple arguments#64410
Jake Bailey (jakebailey) wants to merge 8 commits into
microsoft:mainfrom
jakebailey:inlay-hints-tuple-rest-reuse

Jake Bailey (jakebailey) commented Sep 23, 2026 •

edited

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Jake Bailey (jakebailey) commented Sep 23, 2026 •

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