Dynamic array as a typed constant

[Bogen85]

FPC decided years ago already to break compatiblity with Delphi here to improve type safety. When you have a non-writable constant then it makes sense for it to really be not writable.

I filed an issue on this https://gitlab.com/freepascal.org/fpc/source/-/issues/39986

[ASerge]

When you have a non-writable constant then it makes sense for it to really be not writable.

Then the compiler behaves inconsistently:

Code: Pascal  [Select][+][-]

1. {$MODE OBJFPC}
2. {$WRITEABLECONST OFF}
3. const
4.   A: array[0..0] of SizeInt = (1);
5.   B: array of SizeInt = (2);
6.   C: SizeInt = 3;
7. begin
8.   //A[0] := 5; // Error: Can't assign values to const variable
9.   B[0] := 5;  // No error
10.   //C := 5; // Error: Can't assign values to const variable
11. end.

[Bogen85]

Well, I did file the issue on this https://gitlab.com/freepascal.org/fpc/source/-/issues/39986, but it was immediately closed (not by @PascalDragon).

This is normal. You cannot change the array pointer i_arr, but you can change what it points to.

But you could report a bug for the compiler not rejecting the Inc at compile time. This is likely a remnant of the fact that typed dynamic array constants were only introduced in FPC 3.2.0.

Which is contrary to what you just said...

[avk]

Hehe, looks like MVC is sometimes waist-deep wooden?

[PascalDragon]

FPC decided years ago already to break compatiblity with Delphi here to improve type safety. When you have a non-writable constant then it makes sense for it to really be not writable.

I filed an issue on this https://gitlab.com/freepascal.org/fpc/source/-/issues/39986

Thank you.

When you have a non-writable constant then it makes sense for it to really be not writable.

Then the compiler behaves inconsistently:

As I said: typed dynamic array constants are a fairly recent addition and the compiler has specific checks for various types so it can happen that it behaves inconsistently for them. Which is after all why I had asked Bogen85 to report this.

[Bogen85]

Thank you.

You are welcome. 

So, since constant statically sized array modifications are caught by the compiler:

Code: Pascal  [Select][+][-]

1. program staticarr;
2. {$mode objfpc}
3. {$WriteableConst off}
4.  
5. type
6.   ArrInt3 = array[0..2] of integer;
7.  
8. procedure procarr3(const arr: ArrInt3);
9.   begin
10.         arr[0] := 4;
11.   end;
12.  
13. var
14.   arr3: ArrInt3 = (1,2,3);
15.  
16. begin
17.   procarr3(arr3);
18. end.
19.  

Code: Text  [Select][+][-]

1. Free Pascal Compiler version 3.2.2 [2022/08/31] for x86_64
2. Copyright (c) 1993-2021 by Florian Klaempfl and others
3. Target OS: Linux for x86-64
4. Compiling staticarr.pas
5. staticarr.pas(10,4) Error: Can't assign values to const variable
6. staticarr.pas(19) Fatal: There were 1 errors compiling module, stopping
7. Fatal: Compilation aborted
8. Error: /usr/bin/ppcx64 returned an error exitcode
9.  

Code: Pascal  [Select][+][-]

1. program staticarr2;
2. {$mode objfpc}
3. {$WriteableConst off}
4.  
5. type
6.   ArrInt3 = array[0..2] of integer;
7.  
8. procedure procarr3(var arr: ArrInt3);
9.   begin
10.         arr[0] := 4;
11.   end;
12.  
13. const
14.   arr3: ArrInt3 = (1,2,3);
15.  
16. begin
17.   procarr3(arr3);
18. end.
19.  

Code: Text  [Select][+][-]

1. $ fpc staticarr2.pas
2. Free Pascal Compiler version 3.2.2 [2022/08/31] for x86_64
3. Copyright (c) 1993-2021 by Florian Klaempfl and others
4. Target OS: Linux for x86-64
5. Compiling staticarr2.pas
6. staticarr2.pas(17,16) Error: Can't assign values to const variable
7. staticarr2.pas(19) Fatal: There were 1 errors compiling module, stopping
8. Fatal: Compilation aborted
9. Error: /usr/bin/ppcx64 returned an error exitcode
10.  

And, as I demonstrated earlier:

This does compile and run:

Code: Pascal  [Select][+][-]

1. program dynarr2;
2. {$mode objfpc}
3. {$WriteableConst off}
4.  
5. type
6.   ArrInt = array of LongInt;
7.  
8. procedure procarr(const arr: ArrInt);
9.   begin
10.     arr[0] := 4;
11.   end;
12.  
13. begin
14.   procarr([1,2,3])  
15. end.
16.  

But this fails:

Code: Pascal  [Select][+][-]

1. program dynarr2;
2. {$mode objfpc}
3. {$WriteableConst off}
4.  
5. type
6.   ArrInt = array of LongInt;
7.  
8. procedure procarr(const arr: ArrInt);
9.   begin
10.     arr[0] := 4;
11.   end;
12.  
13. begin
14.   procarr([1,2,3])  
15. end.
16.  

Code: Text  [Select][+][-]

1. Free Pascal Compiler version 3.2.2 [2022/08/31] for x86_64
2. Copyright (c) 1993-2021 by Florian Klaempfl and others
3. Target OS: Linux for x86-64
4. Compiling dynarr2.pas
5. Linking dynarr2
6. 15 lines compiled, 0.1 sec
7.  

Should the above case (that does compile and run) be reported as well? I would indicate it is related to the FPC issue 39986

(as it is a const paramater which might be handled differently?)