Warning C4244 in QStringView.h compare(QChar c)

When compiling a program using Qt on MSVC at /W3 warning level, I get the following error:

C:\Qt\5.15.0\msvc2019_64\include\QtCore/qstringview.h(262): warning C4244: 'return': conversion from 'qsizetype' to 'int', possible loss of data

The line in question is https://github.com/qt/qtbase/blob/1b72f9676f13fc527d26a5b012ef2322414eaa25/src/corelib/text/qstringview.h#L306-L307 (qtbase git master).

class QStringView
{
    ...
    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR int compare(QChar c) const noexcept
    { return empty() || front() == c ? size() - 1 : *utf16() - c.unicode() ; }

This code was only added in Qt 5.15 commit b2f79cc, which explains why I didn't see the warning before. "#include <QWidget>" is sufficient to reproduce this error.

This code is heavily code-golfed. I believe the intent is that "size() - 1" is:

• negative if empty() (aka size() == 0)
• zero if this is a length-1 string equal to c
• positive if it's a long string starting with c.

But if size() is over 2 billion (an extreme edge case), casting qsizetype to int will overflow (which is UB, and may return the wrong value, or return a 64-bit value in a register expected to be 32 bits, or miscompile some other way).

Maybe it should be rewritten and expanded to three if/else statements. If empty(), return -1. If front() == c, return size() > 1. Otherwise return *utf16() - c.unicode().