Qt headers break qdoc on Linux

qdoc seems to break whenever I start using Qt classes on Linux. I am using Ubuntu 64-bit. On Windows 10 64-bit with the same version of Qt it works fine.

Minimum example:

MyClass.h

#pragma once
#include <QString>
struct MyClass { QString foo() { return "Hello World"; } };

MyClass.qdoc

/*!
\module MyModule
*/

/*!
\class MyClass
\inmodule MyModule
\brief Short desc.
*/

/*!
\fn QString MyClass::foo()
Returns "Hello world".
*/

MyClass.module

#pragma once
#include "MyClass.h"

MyClass.qdocconf

outputdir = html
headerdirs = .
sourcedirs = .
headers.fileextensions += *.h
sources.fileextensions += *.qdoc
moduleheader = MyClass.module
includepaths = . $QTDIR/include $QTDIR/include/QtCore

Now this produces following output:

qdoc MyClass.qdocconf [MyClass]

/home/michael/Qt/5.11.2/gcc_64/bin/qdoc /home/michael/qdoctest/MyClass.qdocconf -outputdir /home/michael/build-MyClass-Desktop_Qt_5_11_2_GCC_64bit-Debug/qtc_Desktop_Qt_5_11_2_GCC_64bit_Debug/MyClass.6f532a49/qdoc_html

/home/michael/qdoctest/MyClass.qdoc:5: (qdoc) warning: Cannot find 'MyClass' specified with '\class' in any header file

/home/michael/qdoctest/MyClass.qdoc:11: (qdoc) warning: clang found diagnostics parsing \fn QString MyClass::foo()
    error: use of undeclared identifier 'MyClass'

What happens when I remove the includepaths to Qt includes? The following:

• QString include will be ignored (no warning, nothing)
• Any usage of QString will be replaced with `int`

Well that is not exactly what I would expect but here you go. Anyway, without those inlcude paths and following change to the header:

MyClass.h

#pragma once
#include <string>
struct MyClass { std::string foo() { return "Hello World"; } };

Followed of course by the likewise change to the qdoc file it works fine.

Therefore there must be something in the Qt headers that breaks qdoc on Linux when using GCC 7 and its (system) headers. Or maybe there needs to be some kind of define or environment variable set? This is going on since Qt 5.11 series.