I don't understand.

We're still running this at compile time, so what's the practical difference?

In D28859#649191, @davidedmundson wrote:

I don't understand.

We're still running this at compile time, so what's the practical difference?

The old implement hard-codes it to (minor_version - 7), which is the old contract.

This commit message says it will ties the version with minor version. (I don't think this will change anymore, because patch version will not introduce new API anyway).
https://github.com/qt/qtquickcontrols2/commit/3c7bfc156797b2f3ef08a8cf620b79da80e4b061

In order to cover both cases, this code is doing the check simply checking minor , minor - 1.... until it finds the highest available version.

ping? :)

So the problem we're solving is that we want to go

Qt5.12 - QQC2.5
Qt5.13 - QQC2.6
Qt5.14 - QQC2.7
Qt5.15 - QQC2.15

correct?

We now know all Qt5 versions, and we know that Qt6 won't have versions.

I don't see why need this complicated thing instead of an "if" statement on Qt5QuickControls2_VERSION_MINOR

In D28859#652379, @davidedmundson wrote:

So the problem we're solving is that we want to go

Qt5.12 - QQC2.5
Qt5.13 - QQC2.6
Qt5.14 - QQC2.7
Qt5.15 - QQC2.15

correct?

We now know all Qt5 versions, and we know that Qt6 won't have versions.

I don't see why need this complicated thing instead of an "if" statement on Qt5QuickControls2_VERSION_MINOR

Just to correct it, the change is since 5.12.
5.11 -> 5.4
5.12 -> 5.12