ilya-biryukov added a comment.
This change seems to expose a bug in Clang in C++20 see godbolt
<https://gcc.godbolt.org/z/o8E3nonG8>.
I believe following code should compile, but it does not:
struct X {
virtual ~X();
virtual bool operator ==(X);
bool operator !=(X);
};
struct Y {
virtual ~Y();
virtual bool operator ==(Y);
bool operator !=(Y);
};
struct Z : X, Y {
virtual bool operator==(Z);
bool operator==(X) override;
bool operator==(Y) override;
};
int main() {
bool b = Z() == Z(); // Clang errors saying `lookup for operator != is
ambiguous`
(void)b;
}
The `operator !=` is coming from checking if we need to add a reversed
candidates.
I believe this should compile as according to (over.match.oper)p4
<https://eel.is/c++draft/over.match.funcs#over.match.oper-4>:
> A non-template function or function template F named operator== is a rewrite
> target with first operand o unless **a search for the name operator!= in the
> scope S** from the instantiation context of the operator expression finds a
> function …
And 'search' is defined without any checks for ambiguous bases in
(basic.lookup.general)p3 <https://eel.is/c++draft/basic.lookup#general-3>
> A single search in a scope S for a name N from a program point P finds all
> declarations that precede P to which any name that is the same as N
> ([basic.pre]) is bound in S. If any such declaration is a using-declarator
> whose terminal name ([expr.prim.id.unqual]) is not dependent
> ([temp.dep.type]), it is replaced by the declarations named by the
> using-declarator ([namespace.udecl]).
Clang does a full lookup instead, therefore exposing the errors that it should
not diagnose.
We actually hit this on real code
<https://android.googlesource.com/platform/external/libabigail/+/c0d418081e5cab43d5266ca2e01e6e876b641be3/src/abg-ir.cc#15832>
and found this during integration.
I have sent a fix for review in D157708 <https://reviews.llvm.org/D157708>.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D155387/new/
https://reviews.llvm.org/D155387
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