An accidental attached to a ledger-line note must clear the ledger line stub

A note on or near a ledger line has a short horizontal stub extending past the notehead on both sides (SMuFL legerLineExtension, default ~0.4sp per side); an accidental to that note's left must sit clear of that stub.

A high C6 or low C3 with a sharp sign whose glyph overlaps the ledger line's leftward stub.

SMuFL specification, engravingDefaults.legerLineExtension; Gould, Behind Bars general ledger-line clearance discussion.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.