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

[Michael Van Canneyt]

On Wed, 11 Jan 2023, Bart via fpc-devel wrote:

> 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.