[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.cpp:43
+      Whitelist(
+          utils::options::parseStringList(Options.get("Whitelist", "swap"))) {}
+
----------------
JonasToth wrote:
> JonasToth wrote:
> > logan-5 wrote:
> > > JonasToth wrote:
> > > > do you mean `std::swap`? If you it should be fully qualified.
> > > > Doesn't `std::error_code` rely on adl, too? I think `std::cout <<` and other streams of the STL rely on it too, and probably many more code-constructs that are commonly used.
> > > > 
> > > > That means, the list should be extended to at least all standard-library facilities that basically required ADL to work. And then we need data on different code bases (e.g. LLVM is a good start) how much noise gets generated.
> > > I distinctly //don't// mean `std::swap` -- I want to whitelist any unqualified function call spelled simply `swap`.
> > > 
> > > Overloaded operators are the poster child for ADL's usefulness, so that's why this check has a special default-on `IgnoreOverloadedOperators` option. That whitelists a whole ton of legitimate stuff including `std::cout << x` and friends.
> > > 
> > > I don't see a ton of discussion online about `error_code`/`make_error_code` and ADL being very much intertwined. I'm not particularly familiar with those constructs myself though, and I could just be out of the loop. I do see a fair number of unqualified calls to `make_error_code` within LLVM, though most of those resolve to `llvm::make_error_code`, the documentation for which says it exists because `std::make_error_code` can't be reliably/portably used with ADL. That makes me think `make_error_code` would belong in LLVM's project-specific whitelist configuration, not the check's default.
> > > 
> > > Speaking of which, I did run this check over LLVM while developing and found it not particularly noisy as written. That is, it generated a fair number of warnings, but only on constructs that, when examined closely, //were// a little suspicious or non-obvious.
> > I don't have a solid understanding of the `error_code` world as well. All I know is, that you specialize some templates with your own types in order to use the generic `error_code`-world.
> > AFAIK that needs some form of ADL at some point, but that could even happen through the overloaded operators (`==` and `!=`), in which case that would already be handled. (maybe @aaron.ballman knows more?)
> > 
> > But overloaded operators being ignored by default is good and that point is gone :)
> Yes, `make_error_code` is used via ADL. --> https://www.boost.org/doc/libs/1_72_0/libs/outcome/doc/html/motivation/plug_error_code.html
> I think that should be in the default list for ignored functions, as it is a standard facility.
+1, both `make_error_code` and `make_error_condition` should be on the whitelist. (I am the author of [P0824 "Summary of SG14 discussion of `<system_error>`"](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0824r1.html#best-arthur). I also confirm that libc++ `<system_error>` does call them unqualified on purpose.)

I would like to see some discussion and/or TODO-comments about the other standard [designated customization points](http://eel.is/c++draft/iterator.range#1.sentence-2): `data`, `begin`, `end`, `rbegin`, `rend`, `crbegin`, `crend`, `size`, `ssize`, and `empty`. This might deserve input from the libc++ implementors.


================
Comment at: clang-tools-extra/test/clang-tidy/checkers/bugprone-unintended-adl.cpp:54
+  move(a);
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: expression calls 'ns::move' through ADL [bugprone-unintended-adl]
+  forward(a);
----------------
This is awesome. :)  Please also consider ADL where there's more than one associated namespace, something like this: https://godbolt.org/z/S73Gzy
```
template<class T> struct Templated { };
Templated<std::byte> testX() {
    Templated<std::byte> x;
    using std::move;
    return move(x);  // "correctly" resolves to std::move today, but still does unintended ADL
}
```
Please add a test isomorphic to the above, unless you think it's already covered by one of the existing tests.


================
Comment at: clang-tools-extra/test/clang-tidy/checkers/bugprone-unintended-adl.cpp:61
+void templateFunction(T t) {
+  swap(t, t);
+
----------------
This is not the idiomatic way of calling `swap`: there is no ADL swap for `int`, for example (so `templateFunction<int>` will hard-error during instantiation). It would probably be scope-creep to try to handle the "std::swap two-step", but can you leave a TODO comment somewhere to revisit this issue?



================
Comment at: clang-tools-extra/test/clang-tidy/checkers/bugprone-unintended-adl.cpp:69
+
+  ref(t);
+  ::ref(t);
----------------
Ah, this is subtle. Lookup on `ref` finds the global variable `ref` and therefore doesn't do ADL. But if global variable `ref` didn't exist, then this would warn: `std::ref` is not an ADL customization point.

Could you either leave a explanatory comment here, or (if the subtle confusion with `std::ref` is not germane to the test case) `s/ref/foo/`?


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

https://reviews.llvm.org/D72282