[Development] Qt 6 co-installability with Qt 5

[Shawn Rutledge]

> On 9 Nov 2020, at 19:27, Shawn Rutledge <Shawn.Rutledge at qt.io> wrote:
> 
> 
>> On 2 Nov 2020, at 17:15, Thiago Macieira <thiago.macieira at intel.com> wrote:
>> ]qml is like Python: because of plugins, it's tied to the Qt version. 
>> Therefore, it fails the requirement for supporting everything the old version 
>> supported. 
> 
> Well if you were using modules that are no longer supported, or you run into some little incompatibility; but we have been trying to avoid API breaks.  QML files that begin with “import QtQuick 2.0” still work fine so far; also Controls, Layouts etc.  So IMO it’s less onerous than the python upgrade.

… but your point was not about QML file compatibility but about the mere fact that we have a BC break, so users need two versions of the qml interpreter installed at the same time, right?  And I still rather like the idea of just installing them in different places, and having a symlink to point to the one you want to use.  If distro maintainers insist that /usr/bin/qml must be an executable and not a symlink, then I guess it should be the Qt 6 version, to go along with the fact that we’re pushing the open source community pretty hard to upgrade as soon as it’s released.  The Qt 5 version of qml can be hidden in another directory, because users who are developing new software presumably don’t need it too often anymore.  As for shebang scripts… I will admit that they are probably scarce, because we haven’t publicized this feature much.  And even if some people wrote them, many of those should still work.