[clang-tidy] Check request: modernize-use-std-tie-to-decompose-result

[denzor200]

Needs a check that will find creations of a tuple-like which was assigned from a returned value. The check will show warning to change it to use std::tie.
Also the check will provide fix-it in the case when it's possible to deduce names for future "tied" variables.

BEFORE:

iterator it;
bool succeed;
const auto results = mapping.try_emplace("hello!");      // WARNING AND FIX-IT
it = results.first;
succeed = results.second; // deduced 'it' and 'succeed'

AFTER:

iterator it;
bool succeed;
std::tie(it, succeed) = mapping.try_emplace("hello!"); // use 'it' and 'succeed'

The check will respect original order of default constructors, copy constructors, and assignment operators for a not built-in type and will not provide a fix-it that will broke the order - only warning will be emited:

const auto results = mapping.try_emplace("hello!");      // WARNING
const iterator& it = results.first; // only copy constructor being called
                                    // but with `std::tie` we would call default constructor, copy constructor and then assignment operator
const bool succeed = results.second;

Another example with a built-in type:

const auto pos = get_position();                         // WARNING AND FIX-IT
const float x = pos.first;
const float y = pos.second;

Keep in mind that the check will not provide fix-it in the case when it's impossible to deduce names - just a warning would be enough:

const auto results = mapping.try_emplace("hello!");      // WARNING
if (results.second) {
    handle_inserted(results.first);
}
// no name deduced

In the case when it's impossible to use std::tie no warning must be provided:

const auto results = mapping.try_emplace("hello!");
handle_inserted(results);

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Denis Mikhailov (denzor200)

Needs a check that will find creations of a tuple-like which was assigned from a returned value. The check will show warning to change it to use `std::tie`. Also the check will provide fix-it in the case when it's possible to deduce names for future "tied" variables.

BEFORE:

iterator it;
bool succeed;
const auto results = mapping.try_emplace("hello!");      // WARNING AND FIX-IT
it = results.first;
succeed = results.second; // deduced 'it' and 'succeed'

AFTER:

iterator it;
bool succeed;
std::tie(it, succeed) = mapping.try_emplace("hello!"); // use 'it' and 'succeed'

The check will respect original order of default constructors, copy constructors, and assignment operators for a not built-in type and will not provide a fix-it that will broke the order - only warning will be emited:

const auto results = mapping.try_emplace("hello!");      // WARNING
const iterator& it = results.first; // only copy constructor being called
                                    // but with `std::tie` we would call default constructor, copy constructor and then assignment operator
const bool succeed = results.second;

Another example with a built-in type:

const auto pos = get_position();                         // WARNING AND FIX-IT
const float x = pos.first;
const float y = pos.second;

Keep in mind that the check will not provide fix-it in the case when it's impossible to deduce names - just a warning would be enough:

const auto results = mapping.try_emplace("hello!");      // WARNING
if (results.second) {
    handle_inserted(results.first);
}
// no name deduced

In the case when it's impossible to use std::tie no warning must be provided:

const auto results = mapping.try_emplace("hello!");
handle_inserted(results);