[PATCH] D60570: [Sema] Add more tests for the behavior of argument-dependent name lookup

[Bruno Ricci via Phabricator via cfe-commits]

riccibruno marked an inline comment as done.
riccibruno added inline comments.


================
Comment at: test/CXX/basic/basic.lookup/basic.lookup.argdep/p2-associated-namespaces-classes.cpp:304
+    static_assert(f(g3) == 4, "");        // FIXME: Also well-formed from the union rule.
+                                          // expected-error at -1 {{use of undeclared}}
+  }
----------------
Quuxplusone wrote:
> I see how `g3` matches the example in CWG997
> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#997
> However, I don't see how CWG997's resolution actually affected this example in the slightest. The wording inserted for CWG997 was, "Additionally, if the aforementioned set of overloaded functions is named with a template-id, its associated classes and namespaces are those of its type template-arguments and its template template-arguments." That makes e.g.
> 
>     f(g3<N::S>)
> 
> consider `N::f`, because `N::S` is a "type template-argument" of the template-id `g3<N::S>` which names the set of overloaded functions.  But it doesn't do anything at all to `f(g3)` because `g3` is not a template-id and doesn't have any template-arguments.
> 
> This piece of ADL is implemented only by GCC (not EDG, Clang, or MSVC), and personally I would very much like to keep it that way. We know there's no real-world code that expects or relies on CWG997 — because such code would never work in practice except on GCC. Let's keep it that way!  As soon as you implement a crazy arcane rule, such that code _could_ portably rely on it, code _will start_ relying on it... and then we'll never be able to simplify the language.
> Case in point: the piece of ADL described in this blog post --
> https://quuxplusone.github.io/blog/2019/04/09/adl-insanity-round-2/
> As soon as the above-described arcane ADL rule was implemented in GCC and Clang, Boost.Hana started relying on it; and now the rule is "locked in" to the paper standard because there's real-world code relying on it.
> Personally I'd like to _keep_ real-world code from relying on CWG997, until someone figures out what CWG was thinking when they added it.
I think that the relevant part of CWG 997 is the removal of the restriction on non-dependent parameter types. Sure, `g3` is not a `template-id`, but it refers to an overload set which contains the second `g3`, and one of the parameter of this second `g3` is `N::Q<T>`.

I don't think this is a surprising rule. It matches the general intuition that for function types ADL is done based on the function parameter types and return type. Not having this rule introduces a difference between function templates and functions in overload sets. Consider https://godbolt.org/z/UXHqm2 :
```
namespace N {
    struct S1 {};
    template <typename> struct S2 {};

    void f(void (*g)());
}

void g1();          // #1
void g1(N::S1);     // #2

void g2();                                  // #3
template <typename T> void g2(N::S2<T>);    // #4

void test() {
    f(g1); // ok, g1 is #1
    f(g2); // should be ok, g2 is #3
}
```


Repository:
  rC Clang

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https://reviews.llvm.org/D60570