Generic Dictionary and an inline variable: bug or misuse?

[Kryvich 165]

Hi, I try to adopt outstanding and free Pas2js transpiler to Delphi language. My main development IDE is Delphi CE Rio, so I decided to try a new Delphi syntax: generic collections and inline variables. And stumbled upon a runtime error. The code (simplified):

program TestInlineVarForDictionary;
{$APPTYPE CONSOLE}
{$R *.res}

uses SysUtils, Generics.Collections;

procedure TestDictErr;
var
  Dict: TDictionary<string,TObject>;
begin
  Dict := TDictionary<string,TObject>.Create;
  for var item in Dict do
    Writeln('Key = ', item.Key, 'Name = ', item.Value.ClassName);
end;

procedure TestDictOK;
var
  Dict: TDictionary<string,TObject>;
  item: TPair<string,TObject>;
begin
  Dict := TDictionary<string,TObject>.Create;
  for item in Dict do
    Writeln('Key = ', item.Key, 'Name = ', item.Value.ClassName);
end;

begin
  try
    //!!TestDictOK;
    TestDictErr;
  except
    on E: Exception do begin
      Writeln(E.ClassName, ': ', E.Message);
      Write('Press Enter to continue...');
      Readln;
    end;
  end;
end.

This program causes Exception class $C0000005 with message 'access violation at 0x0040a86e: write of address 0x0040a29e'. Can you confirm it? Is it a bug in the compiler or/and RTL, or am I misusing the new syntax?

It's interesting: if you uncomment TestDictOK that does enumeration in old-style, the exception will disappear!

[Kryvich 165]

Another observation. The exception occurs only if the dictionary has the key and/or value of the string type. If I change it to Integer - the exception disappears.

procedure TestDictErr_IntegerKey_OK;
var
  Dict: TDictionary<Integer,TObject>;
begin
  Dict := TDictionary<Integer,TObject>.Create;
  for var item in Dict do
    Writeln('Key = ', item.Key, 'Name = ', item.Value.ClassName);
end;

Well, I just found it. For procedure TestDictOK the compiler generates a pair of calls:

• call @InitializeRecord
• call @FinalizeRecord

But for procedure TestDictErr it generates only

• call @FinalizeRecord

Program tries to finalize not initialized record and falls.

P.S. Is there a bug bounty program for Delphi? :) 

https://quality.embarcadero.com/browse/RSP-23417

Edited January 25, 2019 by Kryvich

[Kryvich 165]

OK I have a workaround for this issue. Try to specify a type of the inline variable.

procedure TestDictErr_WorkAround;
var
  Dict: TDictionary<string,TObject>;
begin
  Dict := TDictionary<string,TObject>.Create;
  for var item: TPair<string,TObject> in Dict do
    Writeln('Key = ', item.Key, ' Name = ', item.Value.ClassName);
end;

I cannot guarantee that this code will be correctly compiled. But at least there is no the runtime error anymore.

[Stefan Glienke 2002]

This bug has already been reported before but since QP is not responding right now I cannot look for it.