C++20 [[likely]] / [[unlikely]] attribute support

[XuehaiPan]

In C++ attribute: likely, unlikely (since C++20), we can add [[likely]] / [[unlikely]] on if-else clauses. But cpplint does not recognize the [[likely]] / [[unlikely]] attributes after if / else / else if:

Source file:

// foo.cpp
// Copyright 2022 <test>

#include <iostream>

int fib1(int n) {
    if (n > 1) [[likely]] {
        int a;
        return fib1(n - 1) + fib1(n - 2);
    } else if (n == 2) [[unlikely]] {
        int b;
        return 1;
    } else [[unlikely]] {
        if (n == 1) [[likely]]
            return 1;
        else [[unlikely]]
            return 0;
    }
}

int fib2(int n) {
    switch (n) {
        [[unlikely]] case 0: return 0;
        case 1:
            [[fallthrough]];
        case 2:
            [[fallthrough]];
        [[unlikely]] case 3: {
            return 1;
        }
        [[likely]] default:
            return fib2(n - 1) + fib2(n - 2);
    }
}

Command-line:

$ pip3 install -I git+https://github.com/cpplint/cpplint.git
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
Collecting git+https://github.com/cpplint/cpplint.git
  Cloning https://github.com/cpplint/cpplint.git to /tmp/pip-req-build-6nkph3fd
  Running command git clone --filter=blob:none --quiet https://github.com/cpplint/cpplint.git /tmp/pip-req-build-6nkph3fd
  Resolved https://github.com/cpplint/cpplint.git to commit fa12a0bbdafa15291276ddd2a2dcd2ac7a2ce4cb
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: cpplint
  Building wheel for cpplint (setup.py) ... done
  Created wheel for cpplint: filename=cpplint-1.6.1-py3-none-any.whl size=77266 sha256=3e95bbfd3de2b8d61d561339213e6a4ae741e8bb110b34c9b6f626c55cd325c9
  Stored in directory: /tmp/pip-ephem-wheel-cache-d42ge4uo/wheels/af/90/88/663d23197700a259f2a9199aef67ee9977e21f6d1980257857
Successfully built cpplint
Installing collected packages: cpplint
Successfully installed cpplint-1.6.1

$ cpplint foo.cpp
foo.cpp:7:  If/else bodies with multiple statements require braces  [readability/braces] [4]
foo.cpp:10:  If/else bodies with multiple statements require braces  [readability/braces] [4]
foo.cpp:13:  Else clause should never be on same line as else (use 2 lines)  [whitespace/newline] [4]
foo.cpp:13:  If/else bodies with multiple statements require braces  [readability/braces] [4]
foo.cpp:16:  Else clause should never be on same line as else (use 2 lines)  [whitespace/newline] [4]
Done processing foo.cpp
Total errors found: 5

[tkruse]

Than ks for the report, cpplint currently does not support any braking c++20 syntax elements, and I don't know if it ever will (due to lack of maintainers, and googles lack of public cpplint updates).