On the type compatibility in Delphi

Delphi compiler evolves much faster than Delphi documentation, and some language features remain unnamed. Consider the following code snippet:

type
  MyPChar1 = PChar;
  MyPChar2 = type PChar;
  MyPChar3 = ^Char;

procedure Test(Ch: PChar);
begin
end;

procedure TForm1.Button4Click(Sender: TObject);
var
  Ch1: MyPChar1;
  Ch2: MyPChar2;
  Ch3: MyPChar3;

begin
  Test(Ch1);
//  Test(Ch2);     Error E2008 Incompatible types
//  Test(Ch3);     Error E2010 Incompatible types: 'Unit1.Char' and 'System.Char'
end;

The documentation states that MyPChar1 and PChar are identical types, while MyPChar2 and MyPChar3 are distinct; that is why Test(Ch2); and Test(Ch3); lines does not compile. But notice – the compiler issues different error codes. Does it matter?

Consider the next snippet:

procedure Test1(Ch: MyPChar1);
begin
end;

procedure Test2(Ch: MyPChar2);
begin
end;

procedure Test3(Ch: MyPChar3);
begin
end;

procedure TForm1.Button5Click(Sender: TObject);
begin
  Test1('Foo');
  Test2('Foo');   // Compiles
//  Test3('Foo');    Error E2010 Incompatible types: 'MyPChar3' and 'string'
end;

The documentation states that PChar type is assignment compatible on input with string literal. It appears that there is a difference in type ‘distinctness’. The type PChar type is somewhat more compatible with PChar than ^Char type.

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