Strange clangd errors in Qt Creator

[Sevastyan]

I am developing a project using MinGW and qmake with Qt Creator IDE. For some time I have been receiving strange and completely baseless clangd warnings in the QT Creator editor, but the code still compiles correctly. I created a new project to check if I would receive similar warnings and the newly created project works properly without errors. What could I have done wrong in my project that I am getting these errors and how can I fix it? Below are some warnings and a fragment of the .pro file.

.pro file fragment:

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
CONFIG += c++20
CONFIG += qwt
CONFIG += staticlib
CONFIG += static-libgcc
CONFIG += static-libstdc++
CONFIG += disable-desktop
CONFIG += qtquickcompiler

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
CONFIG += c++20
CONFIG += qwt
CONFIG += staticlib
CONFIG += static-libgcc
CONFIG += static-libstdc++
CONFIG += disable-desktop
CONFIG += qtquickcompiler

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Some Qt components include are getting errors, e.g. this:

#include <QWidget>

#include <QWidget>

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is generating this error:

In included file: unexpected type name 'size_t': expected expression

In included file: unexpected type name 'size_t': expected expression

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Also the range based C++11 for loops are generating errors:

for (auto button : buttonGroup_alphabetic.buttons()) {
            button->setText(button->text().toUpper());
        }

for (auto button : buttonGroup_alphabetic.buttons()) {
            button->setText(button->text().toUpper());
        }

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error:

Expected '(' for function-style cast or type construction

Expected '(' for function-style cast or type construction

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