demangle.go
func (st *state) vectorType(isCast bool) AST {
	if len(st.str) == 0 {
		st.fail("expected vector dimension")
	}

	var dim AST
	if st.str[0] == '_' {
		st.advance(1)
		dim = st.expression()
	} else {
		num := st.number()
		dim = &Name{Name: fmt.Sprintf("%d", num)}
	}

	if len(st.str) == 0 || st.str[0] != '_' {
		st.fail("expected _ after vector dimension")
	}
	st.advance(1)

	t := st.demangleType(isCast)

	return &VectorType{Dimension: dim, Base: t}
}
func (st *state) vectorType(isCast bool) AST {
	if len(st.str) == 0 {
		st.fail("expected vector dimension")
	}

	var dim AST
	if st.str[0] == '_' {
		st.advance(1)
		dim = st.expression()
	} else {
		num := st.number()
		dim = &Name{Name: fmt.Sprintf("%d", num)}
	}

	if len(st.str) == 0 || st.str[0] != '_' {
		st.fail("expected _ after vector dimension")
	}
	st.advance(1)

	t := st.demangleType(isCast)

	return &VectorType{Dimension: dim, Base: t}
}
// <vector-type> ::= Dv <number> _ <type>
//               ::= Dv _ <expression> _ <type>