Add new C++ feature to syntax highlighting

syntax/cpp.vim

@@ -37,6 +37,9 @@ syn keyword cppConstant __cplusplus
 
 " C++11
 if !exists('cpp_no_cpp11')
+  " Identifiers with special meaning: override final
+  " TODO: These should not be keyword's, instead highlight based on context
+  " since these can be used as regular identifiers outside of those contexts.
   syn keyword cppModifier override final
   syn keyword cppType char16_t char32_t nullptr_t
   syn keyword cppExceptions noexcept
@@ -51,11 +54,55 @@ if !exists('cpp_no_cpp11')
   " however if the match starts before the beginning of the keyword it will be
   " chosen instead so we can specialze for this case.
   syn match cppStatement '=\s*\(delete\|default\)\>\ze\s*;\?' contains=cOperator display
+
+  " Attribute Specifier Sequence
+  " Match: [[attribute-list]]
+  syn region cppAttributeDelimiter matchgroup=cppDelimiter start='\[\[' skip='\[\[' end='\]\]' transparent contains=cppAttribute,cppNamespace,cppAttributeUsing,cppAttributeUsingNamespace,cDelimiter,cString,cOperator,cNumber,cppFunction,cFunction
+
+  " Standard Attributes
+  syn keyword cppAttribute noreturn carries_dependency contained
 endif
 
 " C++14
 if !exists('cpp_no_cpp14')
   syn match cppNumber display '\<0b[01]\+\(u\=l\{0,2}\|ll\=u\)\>'
+  " Standard Attributes
+  syn keyword cppAttribute deprecated contained
+endif
+
+" C++17
+if !exists('cpp_no_cpp17')
+  " Attribute Specifier Sequence
+  " Match: [[using attribute-namespace: attribute-list]]
+  syn match cppAttributeUsingNamespace '\s\+\zs\w\+\ze\s*:' contained contains=cppAttributeUsing
+  syn keyword cppAttributeUsing using contained
+
+  " Standard Attributes
+  syn keyword cppAttribute fallthrough nodiscard maybe_unused contained
+
+  hi def link cppAttributeUsing cppStatement
+  hi def link cppAttributeUsingNamespace cInclude
+  hi def link cppAttributeUsingDelimiter cppDelimiter
+endif
+
+" C++20
+if !exists('cpp_no_cpp20')
+  syn keyword cppStatement concept requires
+
+  " TODO: Attribute Specifier Sequence
+  " Match: [[contract-attribute-token contract-level(opt) identifier(opt): expression]]
+  "
+  " Standard Attributes
+  syn keyword cppAttribute likely unlikely no_unique_address contained
+  syn keyword cppAttribute expects ensures assert contained
+
+  " Contract Levels
+  syn keyword cppAttribute default audit axiom contained
+  "
+  " TODO: These should not be keyword's, instead highlight based on context
+  " since these can be used as regular identifiers outside of those contexts.
+  " These only occur inside an contracts generalised attribute.
+  syn keyword cppModifier audit axiom
 endif
 
 if !exists('cpp_no_function')
@@ -73,14 +120,14 @@ if !exists('cpp_no_delimiters')
 
   " Match: nested namespace expressions: expr::
   "                                      ^^^^
-  syn match cppNestedName '\w\+::' display contains=cppDelimiter
+  syn match cppNamespace '\w\+::' display contains=cppDelimiter
 
   " Match: label: as a Label and public: protected: private: as a Statement
   "        ^^^^^                 ^^^^^^  ^^^^^^^^^  ^^^^^^^
   syn match cUserCont "^\s*\I\i*\s*:$" contains=cUserLabel,cppAccess display
 
   hi default link cppDelimiter cDelimiter
-  hi default link cppNestedName cInclude
+  hi default link cppNamespace cInclude
 endif
 
 if !exists('cpp_no_operators') && !exists('cpp_no_delimiters')
@@ -114,5 +161,6 @@ hi def link cppConstant Constant
 hi def link cppRawStringDelimiter Delimiter
 hi def link cppRawString String
 hi def link cppNumber Number
+hi def link cppAttribute Keyword
 
 let b:current_syntax = 'cpp'