Only C++17 introduces the rule that static constexpr member variables are implicitly inline.

src/app/puzzle/vptilefactory.cpp

@@ -95,6 +95,11 @@ auto TriangleBasic() -> QPainterPath
 }
 }  // namespace
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+constexpr qreal VPTileFactory::tileStripeWidth;  // NOLINT(readability-redundant-declaration)
+#endif
+
 //---------------------------------------------------------------------------------------------------------------------
 VPTileFactory::VPTileFactory(const VPLayoutPtr &layout, VCommonSettings *commonSettings, QObject *parent):
     QObject(parent),

src/libs/vgeometry/vlayoutplacelabel.cpp

@@ -32,6 +32,12 @@
 #endif
 #include "../ifc/exception/vexception.h"
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+constexpr quint32 VLayoutPlaceLabel::streamHeader;  // NOLINT(readability-redundant-declaration)
+constexpr quint32 VLayoutPlaceLabel::classVersion;  // NOLINT(readability-redundant-declaration)
+#endif
+
 //---------------------------------------------------------------------------------------------------------------------
 VLayoutPlaceLabel::VLayoutPlaceLabel(const VPlaceLabelItem &item)
     : m_center(item.toQPointF()),

src/libs/vlayout/vabstractpiece_p.h

@@ -83,6 +83,12 @@ private:
 
 QT_WARNING_POP
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+constexpr quint32 VAbstractPieceData::streamHeader;  // NOLINT(readability-redundant-declaration)
+constexpr quint32 VAbstractPieceData::classVersion;  // NOLINT(readability-redundant-declaration)
+#endif
+
 // Friend functions
 //---------------------------------------------------------------------------------------------------------------------
 inline auto operator<<(QDataStream &dataStream, const VAbstractPieceData &piece) -> QDataStream &

src/libs/vlayout/vlayoutpiece_p.h

@@ -127,6 +127,14 @@ private:
     static constexpr quint16 classVersion{4};
 };
 
+QT_WARNING_POP
+
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+constexpr quint32 VLayoutPieceData::streamHeader;  // NOLINT(readability-redundant-declaration)
+constexpr quint32 VLayoutPieceData::classVersion;  // NOLINT(readability-redundant-declaration)
+#endif
+
 // Friend functions
 //---------------------------------------------------------------------------------------------------------------------
 inline auto operator<<(QDataStream &dataStream, const VLayoutPieceData &piece) -> QDataStream &
@@ -243,6 +251,4 @@ inline auto operator>>(QDataStream &dataStream, VLayoutPieceData &piece) -> QDat
     return dataStream;
 }
 
-QT_WARNING_POP
-
 #endif // VLAYOUTDETAIL_P_H

src/libs/vlayout/vlayoutpiecepath_p.h

@@ -80,6 +80,12 @@ private:
 
 QT_WARNING_POP
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+    constexpr quint32 VLayoutPiecePathData::streamHeader;  // NOLINT(readability-redundant-declaration)
+    constexpr quint32 VLayoutPiecePathData::classVersion;  // NOLINT(readability-redundant-declaration)
+#endif
+
 // Friend functions
 //---------------------------------------------------------------------------------------------------------------------
 QDataStream& operator<<(QDataStream &dataStream, const VLayoutPiecePathData &path)

src/libs/vlayout/vsapoint.h

@@ -96,6 +96,13 @@ private:
 Q_DECLARE_METATYPE(VSAPoint)  // NOLINT
 Q_DECLARE_TYPEINFO(VSAPoint, Q_MOVABLE_TYPE); // NOLINT
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L
+constexpr qreal VSAPoint::passmarkFactor; // NOLINT(readability-redundant-declaration)
+constexpr qreal VSAPoint::maxPassmarkLength; // NOLINT(readability-redundant-declaration)
+constexpr qreal VSAPoint::minSAWidth; // NOLINT(readability-redundant-declaration)
+#endif
+
 //---------------------------------------------------------------------------------------------------------------------
 Q_DECL_CONSTEXPR inline VSAPoint::VSAPoint(qreal xpos, qreal ypos)
     : VLayoutPoint(xpos, ypos)

src/libs/vpatterndb/vpiecenode_p.h

@@ -116,6 +116,12 @@ private:
     static constexpr quint16 classVersion = 2;
 };
 
+// See https://stackoverflow.com/a/46719572/3045403
+#if __cplusplus < 201703L // C++17
+constexpr quint32 VPieceNodeData::streamHeader;  // NOLINT(readability-redundant-declaration)
+constexpr quint32 VPieceNodeData::classVersion;  // NOLINT(readability-redundant-declaration)
+#endif
+
 // Friend functions
 //---------------------------------------------------------------------------------------------------------------------
 QDataStream &operator<<(QDataStream &out, const VPieceNodeData &p)