Inconsistent use of Qt_6_PRIVATE_API breaks BC between Qt 6.0 and Qt 6.1

Just consider the the output of readelf for the different versions of Qt Network:

${color} readelf -CWs /usr/lib/libQt6Network.so.6.1.0 | grep -i qsslcertificate::frompath
 1888: 00000000000b0d90  2363 FUNC    GLOBAL DEFAULT   11 QSslCertificate::fromPath(QString const&, QSsl::EncodingFormat, QSslCertificate::PatternSyntax)@@Qt_6_PRIVATE_API
${color} readelf -CWs usr/lib/libQt6Network.so.6.0.1 | grep -i qsslcertificate::frompath     
 1813: 00000000000a7890  2363 FUNC    GLOBAL DEFAULT   11 QSslCertificate::fromPath(QString const&, QSsl::EncodingFormat, QSslCertificate::PatternSyntax)@@Qt_6

This leads to failures like

syncthingtray-qt6 --help
syncthingtray-qt6: symbol lookup error: /usr/lib/libsyncthingconnector-qt6.so.1.1.2: undefined symbol: _ZN15QSslCertificate8fromPathERK7QStringN4QSsl14EncodingFormatENS_13PatternSyntaxE, version Qt_6

unless one would recompile the application/library against the new ABI.

It looks like symbols/functions using QSsl::EncodingFormat are affected.

These libraries come from Arch Linux but it looks very similar with the ones provided for openSUSE. Here are the corresponding build scripts:

• https://github.com/archlinux/svntogit-packages/blob/packages/qt6-base/trunk/PKGBUILD
• https://build.opensuse.org/package/show/home:cgiboudeaux:Qt6/qt6-base

I couldn't find any relevant changes (git diff v6.0.0..v6.1.0-beta1 – src/network/ssl/qsslcertificate.{h,cpp}) so I suspect something within the underlying system which makes the decision whether to add Qt_6_PRIVATE_API or not has changed or depends on something random.

I assume the correct behavior is that Qt_6_PRIVATE_API is not added to the symbol name because none of these functions belong to the private API.