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

[Roman Lebedev via cfe-dev]

On Mon, Mar 19, 2018 at 2:40 PM, Krzemienski, Andrzej via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> 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
> };
> ```
FWIW there already is a clang-tidy google-explicit-constructor check,
https://clang.llvm.org/extra/clang-tidy/checks/google-explicit-constructor.html
and one can silence the selected cases with  // NOLINT
And with newest clang-tidy versions support //
NOLINT(google-explicit-constructor)
(See http://clang.llvm.org/extra/clang-tidy/ )

So i may have missed something, but i think the main idea is already
supported, although not as pretty..

> 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
Roman.

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