An anacrusis before a repeat sign is not itself repeated in full bar-length terms — the repeat's internal arithmetic must still close correctly

When a piece opens with an anacrusis and is repeated, the anacrusis is played once at the start; the pickup+final-bar arithmetic (anacrusis-compensating-final-bar) applies to the OVERALL piece's first and last bars, not to each iteration of a repeated inner section, and must not be double-counted at the repeat boundary itself.

A check that naively applies anacrusis-compensating-final-bar's 'last bar must be short by the anacrusis's deficit' rule to the bar immediately BEFORE a repeat-end sign (rather than only the piece's true final bar) would incorrectly demand every repeated section shorten its last bar to match the opening anacrusis — a mistake in the CHECK itself, not the content.

Gould, Behind Bars, ch. 'Bar lines and repeat marks', repeat-sign interaction with pickup measures.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.