cwhy
by Emery Berger, Bryce Adelstein Lelbach, and Nicolas van Kempen
("See why")
Explains and suggest fixes for C/C++/Rust compiler error messages.
Installation
python3 -m pip install cwhy
NOTE: To use cwhy, you must first set up an OpenAI API key. If you
already have an API key, you can set it as an environment variable
called OPENAI_API_KEY. If you do not have one yet,
you can get a key here: https://platform.openai.com/account/api-keys
export OPENAI_API_KEY=<your-api-key>
Usage
Just pipe your compiler's output to cwhy. cwhy will by default provide an explanation. If you'd like a suggested fix, add fix.
e.g.,
% clang++ -g mycode.cpp |& cwhy # explanation only
% clang++ -g mycode.cpp |& cwhy fix # to see a suggested fix
Examples
C++
This highlighted example is missing-hash.cpp, which is one of the first cases we experimented with.
Expand to see the original (pretty obscure) error message:
% clang++ --std=c++20 -c missing-hash.cpp
missing-hash.cpp:13:45: error: call to implicitly-deleted default constructor of 'std::unordered_set<std::pair<int, int>>'
std::unordered_set<std::pair<int, int>> visited;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/unordered_set.h:135:7: note: explicitly defaulted function was implicitly deleted here
unordered_set() = default;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/unordered_set.h:100:18: note: default constructor of 'unordered_set<std::pair<int, int>>' is implicitly deleted because field '_M_h' has a deleted default constructor
_Hashtable _M_h;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:451:7: note: explicitly defaulted function was implicitly deleted here
_Hashtable() = default;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:174:7: note: default constructor of '_Hashtable<std::pair<int, int>, std::pair<int, int>, std::allocator<std::pair<int, int>>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true>>' is implicitly deleted because base class '__detail::_Hashtable_base<pair<int, int>, pair<int, int>, _Identity, equal_to<pair<int, int>>, hash<pair<int, int>>, _Mod_range_hashing, _Default_ranged_hash, _Hashtable_traits<true, true, true>>' has a deleted default constructor
: public __detail::_Hashtable_base<_Key, _Value, _ExtractKey, _Equal,
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1791:5: note: explicitly defaulted function was implicitly deleted here
_Hashtable_base() = default;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1726:5: note: default constructor of '_Hashtable_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Hashtable_traits<true, true, true>>' is implicitly deleted because base class '_Hash_code_base<pair<int, int>, pair<int, int>, _Identity, hash<pair<int, int>>, _Mod_range_hashing, _Default_ranged_hash, _Hashtable_traits<true, true, true>::__hash_cached::value>' has a deleted default constructor
: public _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2, _Hash,
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1368:7: note: explicitly defaulted function was implicitly deleted here
_Hash_code_base() = default;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1344:7: note: default constructor of '_Hash_code_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>' is implicitly deleted because base class '_Hashtable_ebo_helper<1, hash<pair<int, int>>>' has a deleted default constructor
private _Hashtable_ebo_helper<1, _H1>,
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1112:7: note: explicitly defaulted function was implicitly deleted here
_Hashtable_ebo_helper() = default;
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1110:7: note: default constructor of '_Hashtable_ebo_helper<1, std::hash<std::pair<int, int>>, true>' is implicitly deleted because base class 'std::hash<std::pair<int, int>>' has a deleted default constructor
: private _Tp
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/functional_hash.h:101:19: note: default constructor of 'hash<std::pair<int, int>>' is implicitly deleted because base class '__hash_enum<pair<int, int>>' has no default constructor
struct hash : __hash_enum<_Tp>
^
In file included from missing-hash.cpp:1:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/functional:61:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/unordered_map:46:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:35:
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1377:2: error: static assertion failed due to requirement 'std::__is_invocable<const std::hash<std::pair<int, int>> &, const std::pair<int, int> &>{}': hash function must be invocable with an argument of key type
static_assert(__is_invocable<const _H1&, const _Key&>{},
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:1675:29: note: in instantiation of member function 'std::__detail::_Hash_code_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>::_M_hash_code' requested here
__hash_code __code = this->_M_hash_code(__k);
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:788:11: note: in instantiation of function template specialization 'std::_Hashtable<std::pair<int, int>, std::pair<int, int>, std::allocator<std::pair<int, int>>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true>>::_M_emplace<const std::pair<int, int> &>' requested here
{ return _M_emplace(__unique_keys(), std::forward<_Args>(__args)...); }
^
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/unordered_set.h:377:16: note: in instantiation of function template specialization 'std::_Hashtable<std::pair<int, int>, std::pair<int, int>, std::allocator<std::pair<int, int>>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true>>::emplace<const std::pair<int, int> &>' requested here
{ return _M_h.emplace(std::forward<_Args>(__args)...); }
^
missing-hash.cpp:20:44: note: in instantiation of function template specialization 'std::unordered_set<std::pair<int, int>>::emplace<const std::pair<int, int> &>' requested here
const auto [_, inserted] = visited.emplace(n->position);
^
In file included from missing-hash.cpp:1:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/functional:61:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/unordered_map:46:
In file included from /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable.h:35:
/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/hashtable_policy.h:1379:9: error: type 'const std::hash<std::pair<int, int>>' does not provide a call operator
return _M_h1()(__k);
^~~~~~~
3 errors generated.
And here's the English-language explanation from cwhy:
% clang++ --std=c++20 -c missing-hash.cpp |& cwhy
The problem is that you are attempting to use `std::unordered_set`
with `std::pair<int, int>` as the key type. However, the standard
library does not provide a hash function specialization for
`std::pair` out of the box, so the default constructor of the
unordered set is deleted.
To resolve this, you'll need to provide a custom hash function for
`std::pair<int, int>`. Here's an example of how you can define one:
```cpp
struct PairHash {
template <typename T1, typename T2>
std::size_t operator()(const std::pair<T1, T2>& pair) const {
std::hash<T1> hash1;
std::hash<T2> hash2;
return hash1(pair.first) ^ (hash2(pair.second) << 1);
}
};
```
Then, when instantiating the `std::unordered_set`, you can specify the
custom hash function:
```cpp
std::unordered_set<std::pair<int, int>, PairHash> visited;
```
With this change, the code should now compile and work as expected.
Rust
% rustc test/testme.rs |& cwhy
There are three issues: 1. There are two unused variables `x` in the
code. 2. The variable `x` is used after it has already been moved in
the call to `f(x)`, which takes ownership of `x`. 3. The function
`f(x)` takes ownership of `x`, which may not be necessary and could be
changed to borrow the value instead.