[fpc-devel] Why: "Can't take the address of constant expressions" here?

[Bart]

Given the following program (an excerpt form a test program for a
bugreport about the fpwidestring unit):

===
program test;
{$codepage utf8}
{$mode objfpc}
{$h+}

uses
  FpWideString;

var
  WSource: WideString = 'source';
  USource: UnicodeString = 'source';
  WDest: WideString = '' ;
  UDest: UnicodeString = '';
  ASource: AnsiString = 'source';
  ADest: AnsiString = '';
  P: array[0..99] of AnsiChar;

begin
  with WideStringManager do
  begin
    writeln(1);
    Wide2AnsiMoveProc(pwidechar(WSource),RawByteString(ADest),
CP_UTF8, Length(WSource));
    writeln(2);
    Ansi2WideMoveProc(PChar(ASource), CP_UTF8, UDest,
Length(ASource));   //<< test.lpr(24,53) Error: Can't take the address
of constant expressions (caret behind UDest)
end.
====
C:\Users\Bart\LazarusProjecten\bugs\Console\fpwidestring>fpc test.lpr
Free Pascal Compiler version 3.3.1 [2022/10/11] for i386
Copyright (c) 1993-2022 by Florian Klaempfl and others
Target OS: Win32 for i386
Compiling test.lpr
test.lpr(24,53) Error: Can't take the address of constant expressions
...
UDest is of the wrong type here, it compiles fine with WDest (WideString).
I just don't understand the "Can't take the address of constant expressions"
I would have expected something like "Call by var for arg no. 3 has to
match exactly: Got "UnicodeString" expected "WideString""

-- 
Bart