[llvm-bugs] [Bug 31078] New: unexpected "use of overloaded operator '==' is ambiguous"

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31078

            Bug ID: 31078
           Summary: unexpected "use of overloaded operator '==' is
                    ambiguous"
           Product: clang
           Version: 3.9
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: blastrock at free.fr
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following program compiles on clang 3.8, gcc 6.1 and msvc 19 but not on
clang 3.9 and trunk 287442:

    struct Master
    {
    };

    struct A : Master
    {
      using Master::Master;
    };

    bool operator==(A, A) { return true; }

    struct B : Master
    {
      using Master::Master;

      B() {}
      B(A) {}
    };

    bool operator==(B, B) { return true; }

    int main(int argc, char* argv[])
    {
      A a;
      B b;
      a == b;
    }

It says that there are two candidates for the operator== call at the end of
main. Since there is no possible conversion from B to A, the only correct
overload should be the one with B arguments.

I am not sure but it seems that clang accepts constructing an A from a Master.
This line compiles on clang 3.9 but not on gcc 6 and msvc 19:

    A aa = Master{};

I don't know what the standard says and which compiler is correct though...

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