Delphi compatibility: varUInt32 was added but TVarData.varUInt32 was not.
Summary
Delphi added the TVarData.varUInt32 member when varUInt32 was added. To be completely compatible, FPC should add a varUInt32 member to TVarData too. Otherwise projects using TVarData directly will have to distinguish between FPC and Delphi.
Example Project
The Zeos library currently uses the following code:
{$IF Declared(varUInt32)}
TVarData(Result).VType := varUInt32;
TVarData(Result).VUInt32 := FData.pvCardinal;
{$ELSE}
TVarData(Result).VType := varLongWord;
TVarData(Result).VLongWord := FData.pvCardinal;
{$IFEND}
Without this change it doesn't work with FPC trunk.
Relevant 3rd party information
https://docwiki.embarcadero.com/Libraries/Sydney/en/System.TVarData