Despite the clear dis-interest exhibited on the part of the appropriate development/main teams about this issues, I report the continued existence of a persistent challenge when building Qt from source. I do this out of a sense of "civic responsibility", hoping to provide guidance for anyone else who might stumble across this problem. I first reported this for Qt 5.11.0, then for Qt 5.12.0. While I did not attempt to build Qt 5.13.x or 5.14.x, the continued presence in Qt 5.15.0 leads me to presume that it existed in those intervening versions as well.

The problem is exhibited in these 5 files of the source file tree:

...\qtdeclarative\include\QtPacketProtocol\5.15.0\QtPacketProtocol\private\qversionedpacket_p.h
...\qtbase\include\QtFontDatabaseSupport\5.15.0\QtFontDatabaseSupport\private\qwindowsfontdatabase_ft_p.h
...\qtbase\include\QtWindowsUIAutomationSupport\5.15.0\QtWindowsUIAutomationSupport\private\qwindowsuiawrapper_p.h
...\qtbase\include\QtEventDispatcherSupport\5.15.0\QtEventDispatcherSupport\private\qwindowsguieventdispatcher_p.h
...\qtbase\include\QtFontDatabaseSupport\5.15.0\QtFontDatabaseSupport\private\qfreetypefontdatabase_p.h

Each file has a single line of the form:

#include "../../../../../src/plugins/qmltooling/packetprotocol/qversionedpacket_p.h"

The build fails because the compiler cannot find the specified file.

In each case this symptom is resolved by deleting one "../". After doing so, the build completes successfully and produces an apparently fully functional set of Qt libraries.