bug#71784: 31.0.50; Inconsistent fontification for field_identifier in c++-ts-mode

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Wed, Jun 26, 2024 at 06:46:04PM GMT, Eli Zaretskii wrote:
>> Date: Wed, 26 Jun 2024 16:13:47 +0200
>> From:  Ergus via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>> Using the c++-ts-mode I found that there is some inconsistent
>> fontification for the `fields_identifier`:
>>
>> See the fontification in this example with `emacs -Q`.
>>
>> ```test.cpp
>>
>> std::string key;
>> bool inserted;
>>
>> struct name_t {
>> 	std::string key;
>> 	bool inserted;
>> };
>>
>> name_t keys = {"aaa", true};
>>
>> keys.inserted = false;
>> bool a = keys.inserted;
>> ```
>>
>> 1. The `keys.inserted` values are shown differently before or after the
>> = (the inserted word is fontified is some cases, but not in all)
>>
>> 2. The variable names are fontified differently outside or
>> inside the struct.
>>
>> 3. The escape sequence (\t) is fontified differently to the rest of the
>> text inside the string. I don't know if that is intentional or not. If
>> it is intentional, just ignore this comment.
>>
>> The inconsistencies 1 and 2 are not only different to c++-mode but they
>> are semantically incorrect.
>
>What does treesit-explore-mode tell you about these instances of
>keys.inserted?

This is the whole explorer buffer for the example code:

(translation_unit
  (declaration
   type: (qualified_identifier scope: (namespace_identifier) :: name: (type_identifier))
   declarator: (identifier) ;)
  (declaration type: (primitive_type) declarator: (identifier) ;)
  (struct_specifier struct name: (type_identifier)
   body: 
    (field_declaration_list {
     (field_declaration
      type: (qualified_identifier scope: (namespace_identifier) :: name: (type_identifier))
      declarator: (field_identifier) ;)
     (field_declaration type: (primitive_type) declarator: (field_identifier) ;)
     }))
  ;
  (declaration type: (type_identifier)
   declarator: 
    (init_declarator declarator: (identifier) =
     value: 
      (initializer_list {
       (string_literal " (string_content) ")
       , (true) }))
   ;)
  (expression_statement
   (assignment_expression
    left: (field_expression argument: (identifier) operator: . field: (field_identifier))
    operator: = right: (false))
   ;)
  (declaration type: (primitive_type)
   declarator: 
    (init_declarator declarator: (identifier) =
     value: (field_expression argument: (identifier) operator: . field: (field_identifier)))
   ;))

The faces are:

1. Inside the struct insert has: font-lock-property-name-face
    It says `declarator: (field_identifier)` and I thin is applying the
    function c-ts-mode--fontify-declarator.

2. In `keys.inserted = false;` the `insert` words has: font-lock-property-use-face
    It says `field: (field_identifier)` and applies (I think) :feature 'property

3. In `bool a = keys.inserted;` is not fontified.
    But it says `field: (field_identifier)` like in 2.


Hope this helps.
Ergus