error: C2139: 'QString': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to'

[JonB]

@Alcor
In your original code, prior to whatever line causes the error, do you have something like #include <QString> or equivalent other #include which will lead to this?

[Alcor]

@JonB

"In your original code, prior to whatever line causes the error, do you have something like...."

How do I find which line in which file of my code is causing this error?

[SGaist]

@Alcor hi and welcome to devnet,

Where in your code is QString used ?
Do all the files that do something with it contain the proper include ?

[Alcor]

@SGaist

There are many source code files that are using QString such as those given below. But none of these files have directive #include<QString>. I thought adding the line #include<QString> will make the error go away but it unfortunately it stays.

"
foreach(const QString &preset, presets)
{
QStringList temp = preset.split(";");
m_ui->presetComboBox->addItem(temp[0], temp[1] + ":" + temp[2]);
}

"

or
"
void DateTimeWidget::on_utcOffsetDoubleSpinBox_valueChanged(double d)
{
QString prefix("UTC");
if(d >= 0.0)
prefix.append("+");

m_ui->utcOffsetDoubleSpinBox->setPrefix(prefix);
   
autoApply();

}

"

or
"
void CollapsibleDockWidget::updateWindowTitle(QString windowTitle)
{
if(_titleBar)
_titleBar->setWindowTitle(windowTitle);
}

"

[SGaist]

Are you using Qt's QStringBuilder ?

[Alcor]

@SGaist
No. Not to the best of my understanding of the code that I am working with. By the way, if it helps, the code I am working with was originally written (compiled and executed successfully for Qt4.8).

Just now I found one file with include directive #Include <QtCore/QString>.

[kkoehne]

@Alcor said in error: C2139: 'QString': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to':

How do I find which line in which file of my code is causing this error?

Go to the 'Compile Output' pane in Qt Creator. Look for the first "error C2139" . There should be a line with "cl.exe" (the MSVC compiler) before , and that line should contain the name of the source file.

[JonB]

@Alcor
First please follow @kkoehne's suggestion.

It might be time now for you to create a brand new, standalone project in Creator. Keep it tiny, put #include <QString> at the head of the .cpp file and QString temp; somewhere inside a function. As little else as possible. Try to just compile. Does that work or generate errors?

[WWebber]

I ran into exactly the same problem!
Porting from V5.12.x (VS2019) to V6.5.3 (VS2022).
How did you fix this?
The project works perfect with VS2019!
The problem is that the error message does not point to any useful source file than "type_traits".

[aha_1980]

@WWebber: Have you tried the suggestion from @kkoehne in https://forum.qt.io/post/765260 ?

[WWebber]

@aha_1980 said in error: C2139: 'QString': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to':

@WWebber: Have you tried the suggestion from @kkoehne in https://forum.qt.io/post/765260 ?

It points to 'type_traits' and to exactly this line:

struct is_convertible : bool_constant<__is_convertible_to(_From, _To)> {
// determine whether _From is convertible to _To
};

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.43.34808\include\type_traits(323,39): error C2139: 'QString': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to'
1>(compiling source file '../QT/test.cpp')
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qcontainerfwd.h(30,7):
1> see declaration of 'QString'
1> C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.43.34808\include\type_traits(323,39):
1> the template instantiation context (the oldest one first) is
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qstringtokenizer.h(27,30):
1> while compiling class template member function 'QAnyStringView::QAnyStringView(Container &&,wrapped<Container,QString>::type &&)'
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qstringtokenizer.h(27,30):
1> while processing the default template argument of 'QAnyStringView::QAnyStringView(Container &&,wrapped<Container,QString>::type &&)'
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qanystringview.h(216,35):
1> see reference to alias template instantiation 'QAnyStringView::if_convertible_to<QString,QStringTokenizerBaseBase::tokenizer_state&>' being compiled
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qanystringview.h(82,53):
1> see reference to variable template 'const bool conjunction_v<std::negation<std::disjunction<std::is_same<QStringTokenizerBaseBase::tokenizer_state,enum QAnyStringView::Tag>,std::is_sameQStringTokenizerBaseBase::tokenizer_state,QAnyStringView,std::is_pointerQStringTokenizerBaseBase::tokenizer_state,std::is_sameQStringTokenizerBaseBase::tokenizer_state,QByteArray,std::is_sameQStringTokenizerBaseBase::tokenizer_state,QString > >,std::is_convertible<QStringTokenizerBaseBase::tokenizer_state &,QString> >' being compiled
1> c:\Qt6\6.5.3\msvc2019_64\include\QtCore\qanystringview.h(82,53):
1> see reference to class template instantiation 'std::is_convertible<QStringTokenizerBaseBase::tokenizer_state &,QString>' being compiled

[WWebber]

The error shows up with an empty CPP file holding just this single line:
#include <qdialog.h>

[Christian Ehrlicher]

Use a recent Qt version and add the include to QString before.

[WWebber]

@Christian-Ehrlicher said in error: C2139: 'QString': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to':

Use a recent Qt version and add the include to QString before.

This is unfortunately not an option as I write a DLL for a main application using V6.5.3.
Even when I just include <qstring> in the testcpp, I'm gettig the same error.

[Christian Ehrlicher]

Simply try a recent version to see if it works...