// fast path: most likely PosBase hasn't changed
	if m.cache.last == b0 {
		return m.cache.base
	}

	b1, ok := m.bases[b0]
	if !ok {
		fn := b0.Filename()
		absfn := trimFilename(b0)

		if b0.IsFileBase() {
			b1 = src.NewFileBase(fn, absfn)
		} else {
			// line directive base
			p0 := b0.Pos()
			p0b := p0.Base()
			if p0b == b0 {
				panic("infinite recursion in makeSrcPosBase")