Has function initialization code changed from Seattle to Tokyo?

问题

I am in the process of upgrading code from Delphi 10 Seattle to Delphi 10.2 Tokyo and get a lot of H2077 hints Value assigned to ... never used on assignments.
(Even in places where these were explicitly added in the past to get rid of 'may not have a value' warnings).

These are all function initialized like:

Result := 0;
...

Or:

Result := ftType1; // where ftType1 is an enumerated type
...

Did the compiler get smarter in detecting these or has something changed regarding the initial return values of functions?

We have always had these hints 'on', and I always build (not compile).

Example function (1) that builds without hints in Seattle,
but gives the hint H2077 Value assigned to 'GetDatabaseDialect' not used on the first Result := 0 line in Tokyo.

function GetDatabaseDialect(DBName, User, Pswd: string) : integer;
var
  status: array[1..19] of longint;
  szDbName, szDbParam: PANSIChar;
  dbHandle : pointer;
  rslt: longint;

  lDPBBuffer : ANSIString;
  lDPBLength : integer;

  cItem: ANSIChar;
  szRslt: PANSIChar;      //array[0..IBResultBufferSize-1] of ANSIChar;
begin
   Result := 0;
   dbHandle := nil;
   // init database parameter block with version number
   lDPBBuffer := '';
   SetLength(lDPBBuffer, 1);
   lDPBBuffer[1] := ANSIChar(isc_dpb_version1);
   lDPBLength := 1;

   // fill Database Parameter Buffer with user name/password
   lDPBBuffer := lDPBBuffer +
             ANSIChar(isc_dpb_user_name) +
             ANSIChar(Length(User)) +
             ANSIString( User );
   Inc(lDPBLength, 2 + Length(User));

   lDPBBuffer := lDPBBuffer +
             ANSIChar(isc_dpb_password) +
             ANSIChar(Length(Pswd)) +
             ANSIString( Pswd );
   Inc(lDPBLength, 2 + Length(Pswd));

   //Pointers naar naam + buffer
   szDbName  := PANSIChar(ANSISTring(DBName));
   szDbParam := PANSIChar( lDPBBuffer );

   // attach to the database and set dialect
   rslt := isc_attach_database(@status, 0, szDbName, @dbHandle, lDPBLength, szDbParam);
   if rslt <> 0 then
      raise EDatabaseError.Create('Error attaching database!  ISC# ' + IntToStr(rslt));

   //Haal sql dialect op
   szRslt := AllocMem(1000);
   try
      FillChar( szRslt^, 1000, 0);
      cItem := ANSIChar( isc_info_db_SQL_dialect );
      rslt := isc_database_info(@status, @DBHandle, 1, @cItem, 1000, szRslt);
      if rslt <> 0 then
         raise EDatabaseError.Create('Error retrieving database info !  ISC# ' + IntToStr(rslt));


      Result := Ord(szRslt[3]); //3e positie is dialect
   finally
      FreeMem(szRslt);
   end;

   // Drop the connection to the database
   rslt :=  isc_detach_database(@status, @dbHandle);
   if rslt <> 0 then
      raise EDatabaseError.Create('Error detaching database!  ISC# ' + IntToStr(rslt));
end;

Example (2) from a third party library that does not seem to be optimized for Tokyo,
illustrating the case with enumerated types:
H2077 Value assigned to 'TppTemplate.StreamType' not used
Note that changing the assignment to Result := ftASCII; does not make the hint go away (my initial assumption that it was associated with the first enumeration value was incorrect).

type TppFormatType = (ftBinary, ftASCII);

function TppTemplate.StreamType(aStream: TStream): TppFormatType;
var
  lSavePos: Integer;
begin
  {save stream position}
  lSavePos := aStream.Position;
  Result   := ftBinary;  

  try
    ComputeOffsetFromStream(aStream);

    aStream.Seek(FOffset, soBeginning);

    if IsValidASCIISignature(aStream) then
      Result := ftASCII

    else if IsValidBinarySignature(aStream) then
      Result := ftBinary

    else
      raise EInvalidTemplateError.Create(ppLoadStr(49));

  finally
    {restore stream position}
    aStream.Seek(lSavePos, soBeginning);
  end;
end; {function, StreamType}

The common denominator seems to be the Result assignments being in try/finally blocks.

回答1:

Consider this code with a minimal reproduction of your scenario:

function Bar: Boolean;
begin
  Result := Random<0.5;
end;

function Foo: Integer;
begin
  Result := 0;
  if Bar then
    Result := 1
  else
    raise Exception.Create('');
end;

The compiler, even older versions, emits the following hint:

[dcc32 Hint]: H2077 Value assigned to 'Foo' never used

This is reasonable. The first assignment to Result is pointless and can be removed.

Now consider this variation:

function Foo: Integer;
begin
  Result := 0;
  try
    if Bar then
      Result := 1
    else
      raise Exception.Create('');
  finally
  end;
end;

Older versions of the compiler no longer emit the hint, but the latest version of the compiler does. This should be considered a compiler defect, for older versions. The two variants of Foo shown above are semantically identical. The compiler would be justified in generating identical code.

As you surmise, the assignment being inside the try/finally block is necessary to trigger the defect in previous versions.

We can conclude that the Embarcadero developers have fixed a defect in Tokyo. You can resolve the hints by removing the spurious initial assignments.

Of course, if your code is to be compiled by older versions of the compiler, as well as by new versions, then you are in a bind. With the code as it stands now, a hint is emitted by new versions of the compiler. Remove the initial assignment and a hint is emitted by old versions of the compiler.

来源：https://stackoverflow.com/questions/45170376/has-function-initialization-code-changed-from-seattle-to-tokyo