[PATCH] D129951: [clang] teaches Clang the special ADL rules for functions in std::ranges

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


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Comment at: clang/lib/Sema/SemaOverload.cpp:9438
   // FIXME: Pass in the explicit template arguments?
   ArgumentDependentLookup(Name, Loc, Args, Fns);
 
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It would seem preferable to me to do the filtering in `ArgumentDependentLookup` itself rather than here (but I don't feel strongly about it).


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Comment at: clang/lib/Sema/SemaOverload.cpp:9455-9459
+    // Functions in 'std::ranges' are hidden from ADL per [range.iter.ops]/2 and
+    // [algorithms.requirements]/2.
+    if ((*I)->isInStdRangesNamespace() &&
+        Name.getNameKind() == DeclarationName::NameKind::Identifier)
+      continue;
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I worry that this might break some stdlib implementation that finds helper functions via ADL in `std::ranges` somehow. Also, it seems desirable to make this opt-in for the stdlib implementation and indeed for end-user functions not in `std::ranges`.

Have you considered enabling this behavior with an attribute instead of by detecting whether the function is in `std::ranges`?


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Comment at: clang/lib/Sema/SemaOverload.cpp:12837-12841
+  // Functions in 'std::ranges' inhibit ADL per [range.iter.ops]/2 and
+  // [algorithms.requirements]/2.
+  if (!ULE->decls().empty() && ULE->decls_begin()->isInStdRangesNamespace() &&
+      ULE->getName().getNameKind() == DeclarationName::NameKind::Identifier)
+    return;
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What should happen if a `using` declaration names one of these things? Should we care about the properties of the underlying declaration (eg, whether the target of the using declaration is in this namespace / has the attribute), or about the found decl (eg, whether the `using` declaration itself is in the namespace / has the attribute)?

Depending on the answer, we may need to check all declarations in the `UnresolvedLookupExpr`, not only the first one. For example, we could have an overload set that contains both a user-declared function and a `using` declaration that names a function in `std::ranges` here.


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