fix(analyze/types): union static-member flattening #8546

Zaczero · 2025-12-22T02:59:43Z

This PR was written primarily by GPT 5.2.

Summary

Unknown branch propagation

If node might be unknown, node.parentNode must include unknown.

interface Node { parentNode: Node | null }

declare let node: Node | unknown
node.parentNode
// safe approximation: (Node | null) | unknown

Without this, inference can become too narrow by effectively “dropping” the unknown branch

Expression-derived union branches

If a union variant is still an inferred expression type (Biome’s internal “type-of-expression”), flattening obj.member can re-enter expression reasoning in a way that’s unstable for patterns like:

while (node) node = node.parentNode

So we conservatively refuse to flatten in that case (leave it unflattened for this pass).

Probably fixes #8204.

Verified fix on https://github.com/openstreetmap-ng/openstreetmap-ng/blob/main/app/views/lib/standard-form.ts

Test Plan

cargo test -p biome_js_type_info --test flattening
cargo test -p biome_module_graph test_type_flattening_does_not_explode_on_recursive_parent_element_pattern

changeset-bot · 2025-12-22T02:59:48Z

🦋 Changeset detected

Latest commit: 58a37de

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@biomejs/biome	Patch
@biomejs/cli-win32-x64	Patch
@biomejs/cli-win32-arm64	Patch
@biomejs/cli-darwin-x64	Patch
@biomejs/cli-darwin-arm64	Patch
@biomejs/cli-linux-x64	Patch
@biomejs/cli-linux-arm64	Patch
@biomejs/cli-linux-x64-musl	Patch
@biomejs/cli-linux-arm64-musl	Patch
@biomejs/wasm-web	Patch
@biomejs/wasm-bundler	Patch
@biomejs/wasm-nodejs	Patch
@biomejs/backend-jsonrpc	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

coderabbitai · 2025-12-22T03:02:23Z

Walkthrough

This change hardens static-member flattening for union types by switching from a pre-collected variant mapping to an iterative per-variant resolution loop. It skips global-undefined variants, pushes unknown on resolution failures, aborts if a variant is a typeof-expression, and preserves unknown variants in resulting unions. Tests were added to ensure unknown variants are preserved and typeof-expression variants are not flattened. Two new public diagnostic types (JsModuleInfoDiagnostic, ModuleDiagnostic) and a test preventing types-limit explosions for a recursive parentElement pattern were also introduced.

Possibly related PRs

fix(analyze/types): reduce created types when flattening a static member access on a union type #8536: Modifies the same StaticMember-on-Union flattening logic in crates/biome_js_type_info/src/flattening/expressions.rs, switching to per-variant resolution to avoid nested TypeofExpression distribution.

Suggested reviewers

dyc3
ematipico
arendjr

Pre-merge checks and finishing touches

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description thoroughly explains the two key fixes: unknown branch propagation and expression-derived union branches, with concrete examples and test plan.
Linked Issues check	✅ Passed	The PR directly addresses issue #8204 by fixing the type explosion bug in union static-member flattening that occurred with recursive DOM traversal patterns.
Out of Scope Changes check	✅ Passed	All changes align with fixing union static-member flattening: core logic changes, test additions, and new diagnostic type exports are all in scope.
Title check	✅ Passed	The title accurately and concisely summarises the main change: hardening union static-member type flattening to handle edge cases with unknown and inferred expression types.

✨ Finishing touches

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This patch tightens the behavior for TypeofExpression::StaticMember on unions: - If a union variant is unknown, propagate unknown into the result. - Resolve the member across union variants directly (no distribution into nested inferred expressions). - If a union variant is still an inferred expression type (TypeofExpression), refuse to flatten to avoid re-entering expression-based inference that can lead to runaway growth.

dyc3 · 2025-12-22T03:10:33Z

At a glance, this looks a lot like #8536, which is already merged.

Zaczero · 2025-12-22T03:18:54Z

@dyc3 At a glance, yes, and it's because I started working on it yesterday and finished it up after a sleep (at which point this PR was made). But it has 3 benefits in addition to that PR: it avoids vec allocation, correctly handles unknown whereas 8536 stripped unknowns, and it fixes the recursion whereas 8536 did not. The added tests fail on 8536.

codspeed-hq · 2025-12-22T03:29:44Z

CodSpeed Performance Report

Merging #8546 will not alter performance

_{Comparing Zaczero:zaczero/temporary-tortoise (58a37de) with main (6b09620)¹}

Summary

✅ 58 untouched
⏩ 95 skipped²

No successful run was found on main (e8cafcc) during the generation of this report, so 6b09620 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩
95 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. ↩

ematipico

Looks good. Tests were already added, so one isn't needed anymore

ematipico · 2025-12-22T09:15:09Z

crates/biome_js_type_info/tests/flattening.rs

+#[test]
+fn infer_flattened_type_of_static_member_on_union_does_not_flatten_expression_variants() {
+    const CODE: &str = r#"interface Node {
+    parentNode: Node | null;
+}
+
+node.parentNode"#;
+
+    let root = parse_ts(CODE);
+    let interface_decl = get_interface_declaration(&root);
+    let mut resolver = GlobalsResolver::default();
+    let interface_ty =
+        TypeData::from_ts_interface_declaration(&mut resolver, ScopeId::GLOBAL, &interface_decl)
+            .expect("interface must be inferred");
+    resolver.run_inference();
+
+    let interface_ref = resolver.reference_to_owned_data(interface_ty);
+    let expression_variant =
+        resolver.reference_to_owned_data(TypeData::TypeofExpression(Box::new(
+            TypeofExpression::StaticMember(TypeofStaticMemberExpression {
+                object: TypeReference::unknown(),
+                member: Text::new_static("parentNode"),
+            }),
+        )));
+    let union_ty = TypeData::union_of(&resolver, [interface_ref, expression_variant].into());
+
+    let expr = get_expression(&root);
+    let mut resolver = HardcodedSymbolResolver::new("node", union_ty, resolver);
+    let expr_ty = TypeData::from_any_js_expression(&mut resolver, ScopeId::GLOBAL, &expr);
+    let expr_ty = expr_ty.inferred(&mut resolver);
+
+    assert!(
+        matches!(&expr_ty, TypeData::TypeofExpression(expr) if matches!(expr.as_ref(), TypeofExpression::StaticMember(_))),
+        "expected to keep typeof-expression unflattened, got: {expr_ty}",
+    );
+}


This test isn't needed anymore because it was already added

I searched for it but I couldn't find it. The closest was infer_flattened_type_of_static_member_on_union but it checks Node | null instead of the recursive Node | TypeofExpr that this test checks.

dyc3 · 2025-12-22T12:39:36Z

.changeset/harden-union-static-member-flattening.md

The changeset is also not needed, since it was added in the other PR.

Hey, changeset-bot told me the changeset is needed for it to cause a version bump. This is a meaningful improvement, fixing the recursive behavior as shown by the added tests. Can you please explain why it's not needed because I'm getting conflicting information? This is not a noop change.

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

The other PR already added the changeset for the original fix. If the changeset from this PR lands, we will end up with 2 changelog entries for 1 fix.

To clarify, while both PRs touch the same area of the code, they fix slightly different things. The added tests in this PR fail on the original. The original PR did not resolve issue like #8204 on my codebase. They look, at a glance, the same but they are really not. I now see I should have explained it better because I can see how similar and confusing this gets.

Most notable changes are:

fix recursion on typeof expression (original did not)

fix unknown handling (regression after the original)

avoid allocating temporary vec (very minor optimization after the original)

🙂!

Ah, nevermind then. Thanks for clarifying.

github-actions bot added A-Project Area: project L-JavaScript Language: JavaScript and super languages A-Type-Inference Area: type inference labels Dec 22, 2025

Zaczero force-pushed the zaczero/temporary-tortoise branch from 575d3e4 to cdc136e Compare December 22, 2025 03:05

dyc3 requested review from a team December 22, 2025 03:24

Merge branch 'main' into zaczero/temporary-tortoise

58a37de

ematipico approved these changes Dec 22, 2025

View reviewed changes

dyc3 changed the title ~~fix(analyze/types): union static-member flattening~~ refactor(analyze/types): union static-member flattening Dec 22, 2025

dyc3 approved these changes Dec 22, 2025

View reviewed changes

dyc3 changed the title ~~refactor(analyze/types): union static-member flattening~~ fix(analyze/types): union static-member flattening Dec 25, 2025

dyc3 merged commit 0196c0e into biomejs:main Dec 25, 2025
20 checks passed

github-actions bot mentioned this pull request Dec 24, 2025

ci: release #8507

Open

Zaczero deleted the zaczero/temporary-tortoise branch December 25, 2025 15:31

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fix(analyze/types): union static-member flattening #8546

fix(analyze/types): union static-member flattening #8546

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Zaczero commented Dec 22, 2025

Summary

Unknown branch propagation

Expression-derived union branches

Test Plan

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🦋 Changeset detected

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Walkthrough

Possibly related PRs

Suggested reviewers

Pre-merge checks and finishing touches

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dyc3 commented Dec 22, 2025

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Zaczero commented Dec 22, 2025

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CodSpeed Performance Report

Merging #8546 will not alter performance

Summary

Footnotes

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