0022019: Cannot return generic derived class - Free Pascal/Lazarus Bug Tracker

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Project

Category

View Status

Date Submitted

Last Update

0022019

FPC

Compiler

public

2012-05-12 20:10

2012-05-12 20:14

Reporter

Anthony Walter

Assigned To

Priority

normal

Severity

major

Reproducibility

always

Status

new

Resolution

open

Platform

Linux

Ubuntu

OS Version

12.04

Product Version

2.7.1

Product Build

trunk

Target Version

Fixed in Version

Summary

0022019: Cannot return generic derived class

Description

It would seem there is a problem with the compiler when returning a generic class if the class is not specifically referenced elsewhere. See example in steps to reproduce as well as the attached example.

When the error occurs you will see a message like "Error: Forward declaration not solved"

The effect of this problem is that it's not current possible to derive from generic classes unless the derived class has a variable (reference) of it's specific type placed somewhere in the open.

I've encountered a few places where this problem creeps up. One place is in my generic TEnumerator<T> class where I attempt to return a TListEnumerator<T> from a overridden virtual "GetEnumerator: TEnumerator<T>" method. The code will not compile and I am left with the "Error: Forward declaration not solved" message referring to my TListEnumerator<T> methods.

Steps To Reproduce

Example:

type TA<T> = class
public
  function Foo(Value: T): T; virtual; abstract;
end;

type TB<T> = class(TA<T>)
public
  // Error: Forward declaration not solved
  function Foo(Value: T): T; override;
end;

function TB<T>.Foo(Value: T): T;
begin
  Result := Value;
end;

type TC<T> = class
public
  function GetA: TA<T>;
end;

function TC<T>.GetA: TA<T>;
begin
  Result := TB<T>.Create;
end;

procedure Test;
var
  C: TC<String>;
  A: TA<String>;
  // Uncomment this one line below and the error goes away
  // B: TB<String>;
begin
  C := TC<String>.Create;
  A := C.GetA;
  A.Free;
  C.Free;
end;

Additional Information

The problem can be demonstrated to not be "real" by adding a variable of the derived type somewhere. See "uncomment" section in steps to reproduce.

Tags

generics

FPCOldBugId

Fixed in Revision

Attached Files

GenericVirtualTest.pas [^] (1,279 bytes) 2012-05-12 20:10

Relationships

Notes
(0059538) Anthony Walter (reporter) 2012-05-12 20:14 edited on: 2012-05-12 23:43	Click "view advanced" at the top of this page to see the example with steps to reproduce. The example is referenced in my issue description. Small edit: It would seem that the workaround which is to declare variable references to all derived generic types (i.e. uncomment the lines in the examples) needs to be repeated for every pascal unit. If GenericUnit.pas defines "TDerivedGeneric<T> = class(TBaseGeneric<T>)", then any unit which uses GenericUnit will have to create a its own reference to TDerivedGeneric<T>. This means you need to declare "var DummyDerivedGeneric: TDerivedGeneric<Integer>;" or something thereof again and again in every unit or the compiler will stop with "Error: Forward declaration not solved" Another edit: It seems the workaround also fails if the type parameters are different. If you place "var DummyDerivedGeneric: TDerivedGeneric<Integer>;" in your unit, but your actual derive generic is of type TDerivedGeneric<String>, then you'll get the same "Error: Forward declaration not solved" error. Currently to workaround you need a dummy type variable for every derived type multiplied by the number of type variables which you reference multiplied by the number of units using GenericUnit. In short, the workaround doesn't really work.

Issue History
Date Modified	Username	Field	Change
2012-05-12 20:10	Anthony Walter	New Issue
2012-05-12 20:10	Anthony Walter	File Added: GenericVirtualTest.pas
2012-05-12 20:10	Anthony Walter	Tag Attached: generics
2012-05-12 20:14	Anthony Walter	Note Added: 0059538
2012-05-12 20:22	Anthony Walter	Note Edited: 0059538
2012-05-12 21:53	Anthony Walter	Note Edited: 0059538
2012-05-12 21:54	Anthony Walter	Note Edited: 0059538
2012-05-12 21:55	Anthony Walter	Note Edited: 0059538
2012-05-12 23:43	Anthony Walter	Note Edited: 0059538

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