Issue 240507.1: Add support for "properties"

Martin Friebe

## Background

Pascal has a property construct, that allows a "variable like"
identifier, which can either point to a field (member variable) or a
getter/setter function.

    TFoo = class
      FField: integer;
      function GetProp: integer;
      procedure SetProp(AVal: Integer);
      property MyProp: integer read GetProp write SetProp;
      property MyOtherProp: integer read FField;
    end;

* Properties can exist in a structure, interface, or on a global level.

* Properties can be read/write, read only, write only.

* Properties can have one or more array like indexes (of any type).

        function GetProp(AIdx:word; AIdx2: boolean): integer;
        procedure SetProp(AIdx:word; AIdx2: boolean; AVal: Integer);
        property MyProp[AIdx:word; AIdx2: boolean]: integer read GetProp write SetProp;

* Properties can share a method, and provide an index (constant) to
  identify which property called the method.

        function GetProp(AIndex: Integer): integer;
        procedure SetProp(AIndex: Integer; AValue: integer);
        property MyProp1: integer index 1 read GetProp write SetProp;
        property MyProp2: integer index 2 read GetProp write SetProp;

* Properties can have a "default" flag, indicating that the array []
  access can omit the property name. I.e. accessing "Object[n]" is a
  shortcut for the default property. (default for non-array properties is
  being considered for future addition)

* Properties can have "default" and "stored" values for streaming
  (constant or via function).

* Properties can be elevated to a higher visibility (private/public) in
  inherited classes.

There may be partial overlaps with properties in Objective-C and C#.

### References

- [FreePascal property doc][freepascal]
- [Oxygene][oxygene]
- [Apple extension for Objective-C][objc]

[freepascal]: https://www.freepascal.org/docs-html/ref/refse27.html
[oxygene]: https://docs.elementscompiler.com/Oxygene/Members/Properties/
[objc]: https://github.com/llvm/llvm-project/blob/main/llvm/docs/SourceLevelDebugging.rst#debugging-information-format-1

## Proposed Changes

### Section 2.1: The Debugging Information Entry

In Table 2.1, add `DW_TAG_property`, `DW_TAG_property_getter`,
`DW_TAG_property_setter`, and `DW_TAG_property_stored`.

### Section 5.7: Structure, Union and Class Type Entries

Add the following new subsection after Section 5.7.6 Data Member Entries:

> 5.7.x Property Entries
>
> [Non-normative] Many object-oriented languages like Pascal and
> Objective-C have properties, which are member functions that
> syntactically behave like data members of an object. Pascal can also
> have global properties.
>
> A property is represented by a debugging information entry with the
> tag `DW_TAG_property`. A property entry has a `DW_AT_name` string
> attribute whose value is the property name. A property entry may have
> a `DW_AT_type` attribute to denote the type of that property.
>
> A property may have `DW_AT_accessibility`, `DW_AT_external`,
> `DW_AT_virtuality`, `DW_AT_start_scope`, `DW_AT_decl_column`,
> `DW_AT_decl_file` and `DW_AT_decl_line` attributes with the respective
> semantics described for these attributes for `DW_TAG_member` (see Section 5.7.6).
>
> A property may have one or several of `DW_TAG_property_getter`,
> `DW_TAG_property_setter`, or `DW_TAG_property_stored` children to
> represent the getter and setter (member) functions, or the Pascal-style
> `stored` accessor for this property. Each of these tags have a
> `DW_AT_property_forward` attribute to point to a (member) function
> declaration or a data member. If they point to a function, they may also
> have `DW_TAG_formal_parameter` children (matching the ones in the
> function) that can have `DW_AT_default_value` attributes to declare
> additional default arguments for when these functions are used as
> property accessors.
>
> Some languages can automatically derive accessors for properties from a
> field in property's parent object. In such cases the
> `DW_AT_property_forward` attribute of the accessor entry points to the
> `DW_TAG_property`'s sibling `DW_TAG_member` entry of field that holds
> the properties underlying storage.
>
> Property accessors may also have any other attributes allowed in a
> `DW_TAG_subprogram` function declaration. If the value of a property can
> be derived by evaluating a DWARF expression, the

Author:	Martin Friebe
Champion:	Adrian Prantl
Date submitted:	2024-05-07
Date revised:	2025-01-03
Date closed:	2025-03-31
Type:	Enhancement
Status:	Accepted
DWARF version:	6