https://quuxplusone.github.io/blog/2021/01/13/conversion-operator-lookup/

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Arthur O'Dwyer

Stuff mostly about C++

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Fun with conversion-operator name lookup

As of this writing (but perhaps not for very much longer!) the four
mainstream compilers on Godbolt Compiler Explorer give four different
answers for this simple C++ program:

struct A {
    using T = T1;
    using U = U1;
    operator U1 T1::*();
    operator U1 T2::*();
    operator U2 T1::*();
    operator U2 T2::*();
};

inline auto which(U1 T1::*) { return "gcc"; }
inline auto which(U1 T2::*) { return "icc"; }
inline auto which(U2 T1::*) { return "msvc"; }
inline auto which(U2 T2::*) { return "clang"; }

int main() {
    A a;
    using T = T2;
    using U = U2;
    puts(which(a.operator U T::*()));
}

The question is whether U should be looked up in the scope of test or
in the scope of A; and the same question for T.

According to the current draft standard, it sounds like the
conforming answer is "they should both be looked up in the scope of
A"; i.e., GCC's answer is correct and the others are wrong in three
different ways. [basic.lookup.unqual]/5:

    An unqualified name that is a component name of a type-specifier
    or ptr-operator of a conversion-type-id is looked up in the same
    fashion as the conversion-function-id in which it appears. If
    that lookup finds nothing, it undergoes unqualified name lookup;
    in each case, only names that denote types or templates whose
    specializations are types are considered.

I'm never a fan of lookups that don't consider certain kinds of
names; I'm sure there's more divergence to be discovered in this
area. Anyway, in the type name U T::*, U is the type-specifier and
T::* is the ptr-operator, and the whole type is pronounced "pointer
to a data member of T, where that data member itself is of type U."
(More concisely: "pointer to data member (of type U) of T," or
"pointer to a U member of T.")

See also:

  * "Implementation divergence with friend" (2019-09-10)

Posted 2021-01-13
implementation-divergence name-lookup