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Details

Type: Bug
Resolution: Fixed
Priority: P3: Somewhat important
Fix Version/s: 6.4.1, 6.4.3, 6.5.0 FF, 6.5.0, 6.6.0
Affects Version/s: 6.4.0 Beta1, 6.4.2, 6.5.0 Beta1
Component/s: Core: QString and Unicode
Labels:
- reported_by_support_standard
Environment:
MSVC 2019 v16.11.16

Platform/s:

Windows
Commits:
3868f2fa5d42c4d97541b7f9e8a3507eef16008a (qtbase/6.4) 8ba8d1346a562347c398bdd0529d34f94f2ac698 (qtbase/dev), 4bfdf6a3d (dev), 30786ef59 (6.4), a70ebbc67 (6.5)

Description

https://codereview.qt-project.org/c/qt/qtbase/+/399817 introduced a constexpr function, QAnyStringView::isAsciiOnlyCharsAtCompileTime(), to detect ASCII/Latin1 encoding.

Unfortunately, this implementation triggers warning C4702 ("unreachable code") if it is enabled in MSVC, thus blocking compilation when warnings are also treated as errors.

Code

# AnyString.pro
QT -= gui
CONFIG += c++20
SOURCES += main.cpp
QMAKE_CXXFLAGS += -we4702

// main.cpp
#include <QAnyStringView>
#include <QString>

int main() {
    QAnyStringView("hello");
    QAnyStringView(u"hello"_qs);
}

Output

	cl -c -nologo -Zc:wchar_t -FS -Zc:rvalueCast -Zc:inline -Zc:strictStrings -Zc:throwingNew -permissive- -Zc:__cplusplus -Zc:externConstexpr -we4702 -Zi -MDd -std:c++17 -utf-8 -W3 -w34100 -w34189 -w44996 -w44456 -w44457 -w44458 -wd4577 -wd4467 -EHsc /Fddebug\AnyString.vc.pdb -DUNICODE -D_UNICODE -DWIN32 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -DWIN64 -DQT_QML_DEBUG -DQT_CORE_LIB -I..\AnyString -I. -IC:\Qt\6.4.0\msvc2019_64\include -IC:\Qt\6.4.0\msvc2019_64\include\QtCore -Idebug -IC:\Qt\6.4.0\msvc2019_64\mkspecs\win32-msvc -Fodebug\ @C:\Users\Dev\AppData\Local\Temp\main.obj.9408.15.jom
main.cpp
C:\Qt\6.4.0\msvc2019_64\include\QtCore\qanystringview.h(68) : error C4702: unreachable code
C:\Qt\6.4.0\msvc2019_64\include\QtCore\qanystringview.h(69) : error C4702: unreachable code
C:\Qt\6.4.0\msvc2019_64\include\QtCore\qanystringview.h(70) : error C4702: unreachable code
C:\Qt\6.4.0\msvc2019_64\include\QtCore\qanystringview.h(73) : error C4702: unreachable code

Update

The fix at https://codereview.qt-project.org/c/qt/qtbase/+/437324 does not work in C++20 mode because __cpp_lib_is_constant_evaluated is defined in this mode.

For Qt 6.4.2 and Qt 6.5.0-beta1, QAnyStringView("hello"); doesn't trigger the warning but QAnyStringView(u"hello"_qs); does.