CONTRIBUTING.md: update code conventions to C++20

1 files changed, 12 insertions, 10 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c9296df1..3eeac68c 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -155,18 +155,20 @@ just don't bankrupt us with it. Refactoring is welcome.
 
 ### Code style and formatting
 
-As of Quotient 0.6, the C++ standard for newly written code is C++17, with a few
-restrictions, notably:
+As of Quotient 0.6, the C++ standard for newly written code is C++17 with C++20
+compatibility and a few restrictions, notably:
 * standard library's _deduction guides_ cannot be used to lighten up syntax
   in template instantiation, i.e. you have to still write
-  `std::array<int, 2> { 1, 2 }` instead of `std::array { 1, 2 }` or use helpers
-  like `std::make_pair` - once we move over to the later Apple toolchain, this
-  will be no more necessary.
-* enumerators and slots cannot have `[[attributes]]` because moc of Qt 5.9
-  chokes on them. This will be lifted when we move on to Qt 5.12 for the oldest
-  supported version.
-* things from `std::filesystem` cannot be used until we push the oldest
-  required g++/libc to version 8.
+  `std::array<int, 2> { 1, 2 }` instead of `std::array { 1, 2 }` (or use
+  `Quotient::make_array` helper from `util.h`), use `std::make_pair` to create
+  pairs etc. - once we move over to the later Apple toolchain, this will be
+  no more necessary;
+* enumerators and slots cannot have `[[attributes]]` because moc from Qt 5.12
+  chokes on them - this will be lifted when we move on to Qt 5.13 for the oldest
+  supported version, in the meantime use `Q_DECL_DEPRECATED` and similar Qt
+  macros - they expand to nothing when the code is passed to moc.
+* explicit lists in lambda captures are preferred over `[=]`; note that C++20
+  deprecates implicit `this` capture in `[=]`.
 
 The code style is defined by `.clang-format`, and in general, all C++ files
 should follow it. Files with minor deviations from the defined style are still