[rjsgud49]

Summary

Fixes #2141.

merge(value, value) satisfies typeof value started failing in 1.52 when the type contains string & {}, or a union of array shapes such as string[] | string[][] and string | number | T[] | T[][]. toMerged still type-checks, because its return type is T & S.

string & {} counts as an object type, so Merge walked the members of String (length, toString, and the rest). Unions with more than one array shape were merged member by member, which produced a mixed array like (string | string[])[] that is not assignable back to the original type.

Changes

• Return the input type when both sides of Merge are the same type.
• Treat primitives, including branded ones such as string & {}, as values that are replaced rather than merged into.
• Cover the cases from merge() returns an incompatible type when merging two objects of the same type #2141 in the merge and Merge type tests.

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Project	Deployment	Actions	Updated
es-toolkit	Ready	Preview	Oct 19, 2026 4:15am UTC

[raon0211]

Thanks for working on this, @rjsgud49! #2148 was merged yesterday and fixes #2141, so this is already solved on main. It covers the same cases (string & {} and unions of array shapes like string[] | string[][]), and merge(value, value) now returns the input type. Thanks again for the clear write-up of the cause.