feat(analysis): treat subscripted array columns as expressions

[omer-cengel]

Summary

A column reference written with an array subscript or slice, such as labels[1], labels[1:2], or t.labels[1], was analyzed as the whole array column. A projection of it generated a List component and a placeholder compared with it bound a List, both of which compiled but failed against PostgreSQL. Such a reference is now analyzed as an expression, so its type must be stated with a cast.

Changes

• A subscripted or sliced projection is rejected as a computed projection unless a cast states its type: labels[1]::text AS first is a String component, and labels[1:2]::varchar[] AS pair is a list component.
• A subscripted or sliced RETURNING item is rejected as a computed returned item, with or without an alias.
• An uncast placeholder beside a subscripted or sliced reference is no longer typed from the array column. It is reported with the untyped-placeholder diagnostic in comparisons of either order, IN, LIKE, BETWEEN, = ANY, join ON conditions, write WHERE clauses, DO UPDATE ... WHERE, SET assignments, and INSERT values.
• A cast placeholder beside a subscripted reference is typed by its cast and named after itself: an indexed one is param<N>, so labels[1] = $1::text binds param1.
• A subscripted SET or INSERT target must still name an existing column, and the subscript reaches the database as written.
• Query and PostgreSQL array documentation describe the rule, with analyzer tests per location and a PostgreSQL test reading and filtering by one element.

Scope and non-goals

• A subscript is not typed as its element type; a cast states it.
• Unsubscripted column references behave exactly as before, and placeholder binding order stays textual.
• Null tests, join conditions, EXCLUDED values, and conflict targets keep their existing column checks.
• Placeholders inside a subscript, parenthesized or multi-level subscripts such as (labels)[1] and labels[1][2], and open slices are not changed.