feat(lint): detect tuple widening and inferred unions in noMisleadingReturnType

[minseong0324]

I used Claude Code to assist with this implementation.

Summary

Addresses items from #9810: adds tuple element widening and inferred-union handling to noMisleadingReturnType.

Test Plan

Snapshot tests.

Docs

[changeset-bot]

⚠️ No Changeset found

Latest commit: aa326fd

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[codspeed-hq]

Merging this PR will not alter performance

✅ 58 untouched benchmarks
⏩ 196 skipped benchmarks¹

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_{Comparing minseong0324:feat/no-misleading-return-type-tuple-ternary (aa326fd) with main (3d4e70a)}

Footnotes

1. 196 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• .changeset/afraid-items-begin.md

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• .changeset/afraid-items-begin.md

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Walkthrough

This PR extends the noMisleadingReturnType lint rule to detect two additional misleading-return scenarios: tuple element widening (when a const-asserted tuple literal is inferred narrower than a broader annotated tuple) and ternary-inferred unions (when a ternary yields a narrower string-literal union than the annotated return type). The implementation now performs structural tuple element comparisons and validates union variants of inferred types against the annotated type.

Possibly related PRs

• feat(lint): add nursery rule noMisleadingReturnType #9799: Initial implementation of the noMisleadingReturnType rule, which this PR directly extends with tuple and union-handling logic.

Suggested labels

A-Diagnostic

Suggested reviewers

• ematipico
• dyc3

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: detecting tuple widening and inferred unions in the noMisleadingReturnType rule.
Description check	✅ Passed	The description is directly related to the changeset, referencing issue #9810 and explaining the motivation for adding tuple widening and inferred-union handling.

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✨ Finishing Touches

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• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (2)

.changeset/afraid-items-begin.md (1)

5-5: Minor: sentence should end with a full stop.

Per coding guidelines, changeset descriptions should end every sentence with a full stop. Also consider linking to the rule documentation for discoverability.

Suggested fix

-`noMisleadingReturnType` now detects tuple element widening and ternary-inferred unions.
+[`noMisleadingReturnType`](https://biomejs.dev/linter/rules/no-misleading-return-type/) now detects tuple element widening and ternary-inferred unions.

As per coding guidelines: "End every sentence in changeset descriptions with a full stop (period)" and "for rule references, include links to the rule documentation."

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In @.changeset/afraid-items-begin.md at line 5, Update the changeset sentence
for `noMisleadingReturnType` so it ends with a full stop and add a
discoverability link to the rule documentation; specifically, edit the line
containing "`noMisleadingReturnType` now detects tuple element widening and
ternary-inferred unions." to append a period and include a short link/reference
to the `noMisleadingReturnType` rule docs (e.g., in parentheses) immediately
after the sentence.

crates/biome_js_analyze/src/lint/nursery/no_misleading_return_type.rs (1)

811-823: Tuple comparison ignores is_optional and is_rest flags.

Per TupleElementType in crates/biome_js_type_info/src/type_data.rs:915–918, tuple elements have is_optional and is_rest fields. The current pairwise comparison only checks element types and length, so [string?, number] and [string, number] would be treated as structurally compatible. For the as const widening use case this is likely acceptable (const tuples don't typically use optional/rest elements), but worth noting if the rule scope expands.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@crates/biome_js_analyze/src/lint/nursery/no_misleading_return_type.rs` around
lines 811 - 823, The tuple comparison in the TypeData::Tuple arm only compares
element types and lengths, ignoring TupleElementType flags is_optional and
is_rest; update the loop that iterates ann_tuple.elements() and
inf_tuple.elements() to also check these flags for each paired element (e.g.,
require ann_e.is_optional == inf_e.is_optional and ann_e.is_rest ==
inf_e.is_rest, or implement the intended compatibility logic) and return false
on mismatch before resolving types and pushing to stack so tuples like [string?,
number] are not treated as identical to [string, number].

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In @.changeset/afraid-items-begin.md:
- Line 5: Update the changeset sentence for `noMisleadingReturnType` so it ends
with a full stop and add a discoverability link to the rule documentation;
specifically, edit the line containing "`noMisleadingReturnType` now detects
tuple element widening and ternary-inferred unions." to append a period and
include a short link/reference to the `noMisleadingReturnType` rule docs (e.g.,
in parentheses) immediately after the sentence.

In `@crates/biome_js_analyze/src/lint/nursery/no_misleading_return_type.rs`:
- Around line 811-823: The tuple comparison in the TypeData::Tuple arm only
compares element types and lengths, ignoring TupleElementType flags is_optional
and is_rest; update the loop that iterates ann_tuple.elements() and
inf_tuple.elements() to also check these flags for each paired element (e.g.,
require ann_e.is_optional == inf_e.is_optional and ann_e.is_rest ==
inf_e.is_rest, or implement the intended compatibility logic) and return false
on mismatch before resolving types and pushing to stack so tuples like [string?,
number] are not treated as identical to [string, number].

---

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📥 Commits

Reviewing files that changed from the base of the PR and between 8040ec2 and 0014498.

⛔ Files ignored due to path filters (2)

• crates/biome_js_analyze/tests/specs/nursery/noMisleadingReturnType/invalid.ts.snap is excluded by !**/*.snap and included by **
• crates/biome_js_analyze/tests/specs/nursery/noMisleadingReturnType/valid.ts.snap is excluded by !**/*.snap and included by **

📒 Files selected for processing (4)

• .changeset/afraid-items-begin.md
• crates/biome_js_analyze/src/lint/nursery/no_misleading_return_type.rs
• crates/biome_js_analyze/tests/specs/nursery/noMisleadingReturnType/invalid.ts
• crates/biome_js_analyze/tests/specs/nursery/noMisleadingReturnType/valid.ts

[dyc3]

no changeset needed, we haven't released the rule

[minseong0324]

Removed in this commit. aa326fd