for _, pos := range p.plusBuild {
			y, err := constraint.Parse(p.commentTextAt(pos))
			if err != nil {
				x = nil
				break
			}
			if x == nil {
				x = y
			} else {
				x = &constraint.AndExpr{X: x, Y: y}
			}
		}
	case 1:
		// Parse //go:build expression.
		x, _ = constraint.Parse(p.commentTextAt(p.goBuild[0]))
	}

	var block []byte
	if x == nil {