[PATCH] D72282: [clang-tidy] Add `bugprone-unintended-adl`

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone added inline comments.


================
Comment at: clang-tools-extra/clang-tidy/bugprone/UnintendedADLCheck.h:27
+class UnintendedADLCheck : public ClangTidyCheck {
+  const bool IgnoreOverloadedOperators;
+  const std::vector<std::string> AllowedIdentifiers;
----------------
EricWF wrote:
> I think we should always ignore operators. I don't see value in having a mode where every comparison triggers this warning.
> 
I think there's value in that mode, for library writers (not libc++) who really care about finding every unintentional ADL in their whole library. The standard library is designed not-to-work with types like [`Holder<Incomplete>`](https://quuxplusone.github.io/blog/2019/09/26/uglification-doesnt-stop-adl/), but someone else's library might be designed to work even in that case, and then they'd want to hear about ADL lookups for things like `operator,`. Besides, it's just 1 extra line of code in the patch, isn't it?

However, I now think I may not understand how this check works. I thought it looked for unqualified calls (even in templates) that "may" use ADL, but now that I look again at the tests, it seems to trigger only on concrete calls (in concrete template instantiations) that "do" use ADL, which sounds still useful but much less comprehensive than I had thought.

I think it would catch
```
template<class T> void foo(T t) { t, 0; }
struct S { friend void operator,(S, int); };
template void foo(S);
```
but not
```
template<class T> void foo(T t) { t, 0; }
struct S { friend void operator,(S, int); };
template void foo(int);
```
or
```
template<class T> void foo(T t) { t, 0; }
```
is that right?


Repository:
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  https://reviews.llvm.org/D72282/new/

https://reviews.llvm.org/D72282