cwhy 0.2

Explains and proposes fixes for compile-time errors for many programming languages.

cwhy

by Emery Berger, Bryce Adelstein Lelbach, and Nicolas van Kempen.

("See why")

Explains and suggests fixes for compiler error messages for a wide range of programming languages, including C, C++, C#, Go, Java, PHP, Python, Ruby, Rust, and TypeScript.

Installation

python3 -m pip install cwhy

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.

% export OPENAI_API_KEY=<your-api-key>

Usage

Compiler wrapper mode

This new mode is recommended as CWhy will then operate in the same context as the compiler, and will do a better job finding the right source files.

# Invoking the compiler directly.
% `cwhy --wrapper` mycode.cpp

# Using fix mode.
% `cwhy --wrapper fix` mycode.cpp

# Using cwhy with Java.
% `cwhy --wrapper --wrapper-compiler=javac` mycode.java

# Invoking with GNU make, using GPT-4.
% CXX=`cwhy --llm=gpt-4 --wrapper` make

# Invoking with CMake, using GPT-4 and clang++.
% cmake -DCMAKE_CXX_COMPILER=`cwhy --llm=gpt-4 --wrapper --wrapper-compiler=clang++` ...

When running a configuration tool such as CMake or Autoconf, this may greatly increase configuration time, as these tools will occasionally invoke the compiler to check for features, which will fail and invoke CWhy unnecessarily if not available on the machine. To circumvent this, CWHY_DISABLE can be set in the environment to disable CWhy at configuration time.

% CWHY_DISABLE=1 cmake -DCMAKE_CXX_COMPILER=`cwhy --wrapper` ...

Original mode

Just pipe your compiler's output to cwhy.

% clang++ -g mycode.cpp |& cwhy

Options

These options can be displayed with cwhy --help.

• --llm: pick a specific OpenAI LLM. CWhy has been tested with gpt-3.5-turbo and gpt-4.
• --timeout: pick a different timeout than the default for API calls.
• --show-prompt (debug): print prompts before calling the API.

The wrapper mode specifically also has a --wrapper-compiler option to select the underlying compiler to use.

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.