Why the following character pointer types are being considered incompatible? - delphi

Question

Consider the following snippet:
procedure TForm1.FormCreate(Sender: TObject);
{$REGION 'Sealed declarations'}
type WCh = WideChar; // (1)
type Str = ^WCh; // (2)
{ this routine accepts character pointer }
procedure Baz(Param: Str);
begin
end;
{$ENDREGION}
{ this one too, but character pointer type used directly }
procedure Bar(Param: PWideChar);
begin
end;
{ this constant should be compatible with strings and character pointers }
const FOO = 'FOO';
begin
Bar(FOO); // compiles!
Baz(FOO); // BAH! E2010 Incompatible types: 'Str' and 'string'
end;
How do i resolve this problem preserving both structured typing in declarations and the clarity and readability in the usage (i hope for no heavy typecasting)?
NB: By "sealed declarations" i really mean it. I prefer to not amend it unless it is absolutely necessary.
Internal handling of conversion between string and PChar varies from version to version, so environment might matter - i encountered this problem in Delphi XE.

Answer 1

As Rob Kennedy correctly noticed in comments, the question is about conversion from string literal, not string type.
To simplify coding Delphi allows implicit conversion from string literal to PChar type and PChar aliases.
To avoid typecasting you can use
type Str = PWideChar;
or use distict type
type Str = type PWideChar;
I have not noticed any difference in string literal --> PWideChar implicit conversion in Unicode Delphi versions (2009 and above).

Answer 2

Your WCh = WideChar definition creates a type alias for WideChar — they have type identity — but the subsequent Str = ^WCh definition does not create a type alias for PWideChar. When $T+ is in effect, they're compatible and assignment-compatible, but those aren't good enough in this situation. They are still distinct types.
The FOO constant is a string literal. The documentation for assignment compatibility says what types a string literal can be assigned to: "PAnsiChar, PWideChar, PChar or any string type." Str is not a string type. It's a pointer type, but it's not PWideChar, despite how similar their definitions are.
The type of a string literal adapts based on context. When the compiler needs a PWideChar, the string literal is a PWideChar. When the compiler needs an AnsiString, it's an AnsiString. (If the compiler needs both those types, then the literal will be stored in the program both ways.) String literals aren't assignable to your Str type, so, according to the error message, the compiler apparently chooses string as the type for the string literal in that situation. You can type-cast it to one of the other built-in types, but the better solution would be to avoid using custom-defined character-pointer classes at all.