[Development] Proposing CMake as build tool for Qt 6

[Thiago Macieira]

On Thursday, 13 June 2019 01:06:06 PDT Bogdan Vatra via Development wrote:
> Hi,
> 
>   There is one more missing feature to add to your list: build & debug
> builds for msvc in one go (same as qmake does now, a single "make" command
> will build both targets).

I wouldn't call that a hard requirement. Separate builds for debug and for 
release are acceptable, the same way that we've ostensibly had support for 
shared-and-release builds but in practice they're always built separately.

The only difficult part I see is enforcing the suffix. On Mac we've had 
problems with this, since a debug-only build is un-suffixed. Maybe it's time 
to rethink the strategy and make the suffix optional altogether (using the lib 
infix feature). On Windows, we should stop enforcing that unoptimised builds 
be -MDd and make it easy for people to debug into -MD libraries. Hardly anyone 
ever debugs into Microsoft's libraries...

>   Also I wonder when do you plan to release Qt6? Because according to
> Thiago's "what a buildsystem must to have list" he mentioned that the
> buildsystem should be at least 2 years in the wild. If the needed cmake
> support is not implemented yet, are you going to delay Qt 6 release (I
> won't mid to use C++20 in Qt 6 :) ) just to give time to cmake to mature?
> Or that (part of the) list was just an excuse for TQC to ditch QBS ;-).

2 years in the wild was not for all features, it was for the buildsystem 
itself and the major features. But we need to build with the version that 
comes with the host systems that we are targetting (if they ship tools like 
this in the first place; Windows doesn't).

That means that the iOS support only needs to be present in whatever version 
of CMake that Homebrew installs. Since Homebrew always has the latest, this is 
not an impediment.

But if we want to build on Ubuntu 16.04, then that three year old version's 
CMake needs to be sufficient for a native build of Qt and whatever cross-
builds get targeted from Linux (Android and QNX, I'd guess). For Qt 6, I'd 
just recommend raising the minimum to 18.04...

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel System Software Products