fixing compilation issue: value usage of fwd declared struct

[PaulXiCao]

This addresses issue #1822.

The issue has been using a vector where the underlying value type has only been forward declared and defined later on, e.g.

struct NamedOptimizationMethod;
std::vector<std::vector<NamedOptimizationMethod> > optimizationMethods_;
struct NamedOptimizationMethod {
        ext::shared_ptr<OptimizationMethod> optimizationMethod;
        std::string name;
};

Minimal (non-)working example: godbolt

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.

[CLAassistant]

All committers have signed the CLA.

[lballabio]

Thanks! May you have a look at the CLAssistant comment above?

[coveralls]

coverage: 72.016%. remained the same
when pulling 0cdb45d on PaulXiCao:iss1822_clang20
into 62c0f14 on lballabio:master.

[PaulXiCao]

Thanks! May you have a look at the CLAssistant comment above?

Hey @lballabio, I just signed the license agreement.

Though I am unsure if the commit user name and email are correctly linked to this GitHub account (and viewable by the CLAssistant). I will just wait and see if the bot interacts with this mr again.

[lballabio]

Hmm, you're right, the email associated to the commit is invalid (it's <{ID}+{username}@users.noreply.github.com>).
May I trouble you to force-push a new commit with a valid address, or to open a new PR? Apologies but we need to be strict about licensing issues.

[PaulXiCao]

Totally understand the license thingy. I will fix it on my side, edit the commits and force push.

Edit: All done now.