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""

I think that what happens is that the compiler sees that it can convert widestring to unicodestring and allows a conversion, so it inserts a conversion node, but the result of the conversion is a constant expresson so it generates an error in a later stage of compilation.

But my knowledge is limited, someone else will need to confirm this.

Michael.

First of all, adding a 'end;" for the "with" compiles under Linux. That's because widestring=unicodestring on Linux.

On Windows widestring is not a unicodestring and thus it can't be passed to a var parameter.

https://wiki.freepascal.org/Widestrings

Mattias

The issue is indeed the conversion from UnicodeString to WideString which is not allowed for a var/out parameter. That the compiler doesn't use the error “Call by var for arg no. 3 has to match exactly: Got "UnicodeString" expected "WideString"” is due to Ansi2WideMoveProc() being a function pointer. The code that determines that error is not called in that case, instead the other is caused essentially as a last resort.

You can report a bug with the following sample to hopefully improve this:

=== code begin ===

program tunicode;
var
   Str: UnicodeString = 'Foobar';
procedure Test(var aArg: WideString);
begin
end;
var
   Func: procedure(var aArg: WideString) = Nil;
begin
   Test(Str);
   Func(Str);
end.

=== code end ===

Regards,
Sven

Thanks for explaining.

You can report a bug with the following sample to hopefully improve this:

Reported as issue #40106

Thank you.

Regards,
Sven

Mattias Gaertner wrote

On Wed, 11 Jan 2023 23:58:34 +0100 Bart via fpc-devel wrote:

[...]

i'm a little surprised there is no "mismatched begin/end" error with the caret pointing to the 2nd begin...

That's a copy/paste error.by me.
The original code had about 10 more function calls and then a closing end for the "with WideStringManager do begin "

oh! that would explain it :lol: