As a developer of Qt, I'd like to have guidance on how to properly document constrained templates

Constrained templates are those that either use std::enable_if (openly, or hidden in a if_x<T> = true template alias), or, come C++20, requires clauses.

We need to make sure that qdoc understands these enable_if and later requires clauses and produces sensible documentation from them.

Currently, we either ignore the issue and hope that qdoc will produce something sensible from the C++ declaration, or we fake some unconstrained versions of the function under #ifdef Q_QDOC. The former obviously works in that qdoc copied the enable_if verbatim (https://doc.qt.io/qt-6/qstringview.html#QStringView-3), but since the constraint isn't clickable, the user is left wondering what the heck that thing is. The latter is a lie to the user, and means she needs to look into the header instead of the docs when some overloading/SFINAE goes wrong for her.

So, we need to up our game here.

Please come up with a way to document concepts (not just the C++20 ones, which are a ways out, still, but also incidental C++17 ones like QStringView's if_compatible_char), get stakeholder approval, and implement what's missing in qdoc to support it.