Handle Array slice assignment (ary[start, len] = val) in builtin#418
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Handle Array slice assignment (ary[start, len] = val) in builtin#418
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Previously, Array#[]= with 3 arguments (slice assignment) fell through to RBS overload resolution, where both (int, int, Elem) and (int, int, Array[Elem]) overloads matched. The Elem overload caused the assigned array to be treated as an element, creating deeply nested union types like Array[Array[Array[Integer]]]. Now the builtin handles 3-argument []= by using SplatBox to extract element types from the assigned value and merge them into the array's element type, preventing type nesting. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Previously, Array#[]= with 3 arguments (slice assignment) fell through to RBS overload resolution, where both (int, int, Elem) and (int, int, Array[Elem]) overloads matched. The Elem overload caused the assigned array to be treated as an element, creating deeply nested union types like Array[Array[Array[Integer]]].
Now the builtin handles 3-argument []= by using SplatBox to extract element types from the assigned value and merge them into the array's element type, preventing type nesting.