We're reading from disk, so we go from type in disk to type in destination
The type of destination is SomeVectorType
The underlying type of destination is SomeVectorType::value_type
Therefore, we have underlying type in disk and underlying type in destination
Let's call floating the set {double, float}
underlying type ut_disk in disk is in {floating, complex, other}
underlying type ut_dest in destination is also in {floating, complex, other}
if ut_disk == ut_dest all is good
if ut_disk != ut_dest we throw unless
ut_disk == floating and ut_dest is std::complex<floating> so
that we can go from floating to complex<floating>

I won't talk too much about implementation because it might change.
Note the complication of taking Real<SomeVectorType::value_type>::Type
which is not discussed here because
"The source is fully true." (Gonzalo, 2021)

As far as disk is concerned, a name + "ComplexOrRealType" single char set
is written to the hd5 file only if
vector<floating>, with content 'R'
vector<std::complex<floating> >, with content 'C'
all other vectors aren't prepended at all and
a read for name + "ComplexOrRealType" set in the hd5 file will fail.