llvm/llvm-project

[clang] [C++20] Defaulted `operator==` doesn't lookup in using-directive properly

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#97,087 opened on Jun 28, 2024

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clang:frontendgood first issue

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Description

https://godbolt.org/z/c8v65nGzc

struct X {};
namespace NS {
    bool operator==(X, X);
}
using namespace NS;
struct Y {
    X x;
    friend bool operator==(Y, Y);
};
bool operator==(Y, Y) = default;

<source>:10:6: error: defaulting this equality comparison operator would delete it after its first declaration
   10 | bool operator==(Y, Y) = default;
      |      ^
<source>:7:7: note: defaulted 'operator==' is implicitly deleted because there is no viable 'operator==' for member 'x'
    7 |     X x;
      |       ^

If it is defaulted in the class it does compile and properly call NS::operator==(X, X): https://godbolt.org/z/M8Msc8T3a
If the function-body is replaced it also compiles: https://godbolt.org/z/19sEre7KW

Contributor guide

Research direction
Examine how the compiler resolves operator== for a defaulted definition outside the class, focusing on the using directive lookup path. Compare with the in class definition case.
Tech stack
cpp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50