Use templates without constexpr

std::variant with std::visit might do the dispatch:

using cudadatatypevar = std::variant<
    std::typeidentity,
    std::typeidentity,
    std::typeidentity,
    std::typeidentity
>;
cudadatatypevar selectcudadatatype(MYDatatype dtype) {
    switch (dtype) {
        case MYF32: return std::typeidentity{};
        case MYF64: return std::typeidentity{};
        case MYC32: return std::typeidentity{};
        case MYC64: return std::typeidentity{};
    }
    throw std::runtimeerror("Unsuported value");
}
void dispatch(cudadatatypevar var1,
              cudadatatypevar var2,
              cudadatatypevar var3)
{
    std::visit([](std::typeidentity,
                                                         std::typeidentity,
                                                         std::typeidentity) {
            sometemplatefunction();
        }, var1, var2, var3);
}
void dispatch(MYDatatype t1, MYDatatype t2, MYDatatype t3)
{
    dispatch(selectcudadatatype(t1),
             selectcudadatatype(t2),
             selectcudadatatype(t3));
}

Demo