[llvm-bugs] [Bug 50239] New: member using declaration of function templates with different return types does not bring base class overload into derived class

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50239

            Bug ID: 50239
           Summary: member using declaration of function templates with
                    different return types does not bring base class
                    overload into derived class
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalo.gadeschi at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Clang incorrectly accepts the following example
(https://clang.godbolt.org/z/7abPjfcf6):

template <class T> struct A {
  template <class U> long f(U) const { return 1; }
};
struct B : A<int> {
  using A<int>::f;
  template <class U> int f(U) const { return 2; }
};
int main() {
  return B{}.f(3);
}

Instead, it should error at "B{}.f(3)", since the call is ambiguous. 

A member using declaration brings into the derived class all the overloads of
the specified function from the given base class, except for any overloads
where the derived class has a function declaration with the same signature
([namespace.udecl]/p11).

For non-template functions, the return type is not part of the signature
([defns.signature.member]).

For function templates, the return type is part of the signature
([defns.signature.member.templ]).  

In this example, A<int>::f and B::f have different return types (long vs int),
and have different signatures. B::f does not hide A<int>::f, and both B::f and
A<int>::f are available for overload resolution. 

Because they have the same parameters types, any call is ambiguous.

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