QKeySequence::toString() returns garbage strings for some simple QKeySequence

Summary:

QKeySequence::toString() returns garbage strings for some simple QKeySequence like Qt::Key_Shift, Qt::Key_Control, Qt::Key_Mode_switch, ... The issue can be reproduced on macOS 10.13.6 as well as on Windows 10.

Steps to reproduce (macOS):

1. Download the attached application GarbageKeySequenceDescription: GarbageKeySequenceDescription.zip
2. Compile the application on macOS 10.13.6 and run it

Result: You see garbage strings for Key_Shift, Key_Control and Key_Mode_switch:

Key_Left: 
portableKey: "Left"
nativeKey: "←"

Key_Shift: 
portableKey: "ៀ\uDC20"
nativeKey: "⇧"

Key_Control: 
portableKey: "ៀ\uDC21"
nativeKey: "⌘"

Key_Mode_switch: 
portableKey: "ោ\uDD7E"
nativeKey: "ោ\uDD7E"

Steps to reproduce (Windows 10):

1. Download the attached application GarbageKeySequenceDescription: 
2. Compile the application on Windows 10 and run it

Result: You see garbage strings for Key_Shift, Key_Control and Key_Mode_switch:

Key_Left: 
portableKey: "Left"
nativeKey: "Left"

Key_Shift: 
portableKey: "?\uDC20"
nativeKey: "?\uDC20"

Key_Control: 
portableKey: "?\uDC21"
nativeKey: "?\uDC21"

Key_Mode_switch: 
portableKey: "?\uDD7E"
nativeKey: "?\uDD7E"

More information:

The code simply does the following:

QString nativeKey = QKeySequence(Qt::Key_Shift).toString(QKeySequence::NativeText);
qDebug() << "nativeKey: " << nativeKey;

Expected: 

QKeySequence::toString() should never return a garbage string. It should return an empty string if it can't provide a valid description.