[cfe-dev] Add a clang-tidy check for inadvertent conversions

[Krzemienski, Andrzej via cfe-dev]

Hi Everyone,
I am considering contributing a clang-tidy check that will also require changes to clang sources. I would like to solicit feedback from people in this list. Here is the idea.

The goal is to detect constructors unintentionally defined as converting (when someone has forgotten to declare one's constructor as explicit).
I want to add an attribute, call it, say, [[conversion]] or [[implicit]]. It applies only to constructors. I want to add another check to clang tidy, say cppcoreguidelinse-explicit-ctor (referring to http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-explicit). 
The warning is triggered when:

1. We see a declaration of non-copy, non-move constructor that can be called with a single argument and it is neither declared explicit not annotated with attribute [[conversion]].

2. We see a declaration of a copy ctor or a move ctor or a constructor that cannot be called with a single argument which annotated with attribute [[conversion]].

Example:

```
Struct X
{
  X(X const&); // ok: copy-ctor is special
  X(int I, int j, int k); // ok: cannot be called with one arg
  X(int i, int j = 0); // warn: non-copy, non-move, non-explicit, can be called with one arg
  template <typename... T> X(T&&... args); // warn: can be called with 1 argument
  explicit X(double); // ok: explicit
  X(char) [[conversion]]; // ok: warning disabled by attribute

  explicit X(std::string) [[conversion]]; // warn: attribute is redundant
  X(char, char) [[conversion]]; // warn: attribute is redundant
};
```

Proposed implementation:
Currently the AST matchers are not capable of extracting information from user-defined attributes. Implementing such extraction is beyond my capacity. Instead, I could teach clang to recognize one more attribute, this [[conversion]], the same way as it recognizes [[noreturn]]. Hen adding a clang-tidy check would be quite trivial.

What do you think about it?

Regards,
Andrzej Krzemienski