fix(utils): wrap discriminator merge check in parentheses to fix precedence

[greymoth-jp]

Backports #16187 to the 7.x line.

utils.merge() guards against merging a single-nested path against a document-array path (and vice versa) when building a discriminator schema (gh-9534). Because && binds tighter than ||, the condition

options.isDiscriminatorSchemaMerge &&
(from[key].$isSingleNested && to[key].$isMongooseDocumentArray) ||
(from[key].$isMongooseDocumentArray && to[key].$isSingleNested)

parses as (A && B) || C, so the continue also fires when isDiscriminatorSchemaMerge is false and from is a document array while to is single-nested. The path is then silently dropped from a normal (non-discriminator) merge. Wrapping the two clauses restores A && (B || C).

The 7.x utils.merge() body is identical to master apart from this grouping, so the fix and its test port over unchanged.

Verified on 7.x: the isDiscriminatorSchemaMerge: false test fails before the change (undefined !== 2) and passes after; test/utils.test.js stays green (26 passing).

[Copilot]

Pull request overview

Backports a fix to utils.merge() in the 7.x line to ensure the discriminator-specific schema merge guard is correctly gated by options.isDiscriminatorSchemaMerge, preventing unintended key skipping in normal merges.

Changes:

• Fix JavaScript operator-precedence in utils.merge() by wrapping the discriminator mismatch check in parentheses.
• Add regression tests ensuring the mismatch-skip behavior only applies when isDiscriminatorSchemaMerge: true.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/utils.js	Wrapes the single-nested vs document-array mismatch condition so it’s fully guarded by isDiscriminatorSchemaMerge.
test/utils.test.js	Adds regression coverage for both discriminator and non-discriminator merge behavior for the mismatch case.

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