Type inference for character literals (and HIGHCHARUNICODE handling)

[zaneduffield]

Prerequisites

• [ ] This improvement has not already been suggested.
• [x] This improvement would be generally useful, not specific to my code or setup.

Engine area

Delphi language support

Improvement description

Related to #116 is the problem of type inference of character literals.

The page on the HIGHCHARUNICODE directive provides a pretty good explanation of how it's supposed to work

The {$HIGHCHARUNICODE ON} directive controls the behavior of characters #$80 ... #$FF (#⁠128 ... #⁠255).

When HIGHCHARUNICODE is OFF:

• All decimal #xxx n-digit literals are parsed as AnsiChar.
• All hexadecimal #$xx 2-digit literals are parsed as AnsiChar.
• All hexadecimal #$xxxx 4-digit literals are parsed as WideChar.

When HIGHCHARUNICODE is ON:

• All literals are parsed as WideChar.

This tells us that the type of a character literal depends both on its value and the state of the HIGHCHARUNICODE directive.

Here's a short example that illustrates how this could make a difference to analysis

{$HIGHCHARUNICODE OFF}

procedure Foo(a: AnsiChar); overload;
begin
end;

procedure Foo(a: WideChar); overload;
begin
end;

begin
  Foo(#128); // calls Foo(AnsiChar) (when HIGHCHARUNICODE is OFF)
  Foo(#127); // calls Foo(WideChar)
end.

Rationale

As in #116, this is required for fully accurate type resolution.