Historically in Delphi, the Char type was effectively a synonym for the ANSIChar type. That is, a single byte representing a character from an ANSI codepage. NOTE: This is a simplification that ignores the complications arising from multibyte characters which could be encountered in an ANSI string but will suffice for this answer.
This corresponded with the fact that the String type was effectively a synonym for ANSIString.
In Delphi 2009 onward, this changed.
With Delphi 2009, the String and Char types became synonyms for UnicodeString (a WideString with additional capabilities) and WideChar, respectively, reflecting the transition to Unicode as the native format for string and character types. A WideChar is a 2 byte value representing a single character of Unicode (or one half of a surrogate pair).
Therefore, in versions of Delphi prior to Delphi 2009, the following two variables were of compatible types:
var
ach: ANSIChar;
ch: Char; // Synonymous with ANSIChar
However, in Delphi 2009 and later the meaning of the "ch" declarations changes:
var
ach: ANSIChar;
ch: Char; // Synonymous with WIDEChar
As a result, the ach and ch variables are no longer of compatible types.
i.e. the reason you are getting this error is that you have some code which has been declared with ANSIChar types and other code which is using values declared of type Char. When compiled with an old version of Delphi where Char = ANSIChar, the two sets of code are compatible, but in Delphi 2009 and later Char = WideChar and so the two types (Char and ANSIChar) are not compatible.
