[llvm-bugs] [Bug 38237] New: Default argument checking not performed after overload resolution with C linkage

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38237

            Bug ID: 38237
           Summary: Default argument checking not performed after overload
                    resolution with C linkage
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zhonghao at pku.org.cn
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

he following code is ill-formed according to the standard, but accepted by
clang++:

namespace A {
        extern "C" void f(int = 5);
}
namespace B {
        extern "C" void f(int = 4);
}

using A::f;
using B::f;
int main() {
    f(3);    //OK
    f();     //ERROR
}

When a default argument is used as in the second call, it needs to be looked up
to make sure it isn't declared twice. If it is, then it's an error. clang++
will also accept the code whether the default arguments to f() are given the
same or different values.

Instead, g++ rejects the code:

code0.cpp:12:4: error: default argument mismatch in overload resolution
[-fpermissive]
  f(); //ERROR
    ^
code0.cpp:2:18: note:  candidate 1: 'void A::f(int = 5)'
  extern "C" void f(int = 5);
                  ^
code0.cpp:5:18: note:  candidate 2: 'void B::f(int = 4)'
  extern "C" void f(int = 4);
                  ^

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