[Bug c++/71534] New: Initializing a static constexpr data member of a base class by using a static constexpr data member of a derived class should be an error

["michele.caini at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71534

            Bug ID: 71534
           Summary: Initializing a static constexpr data member of a base
                    class by using a static constexpr data member of a
                    derived class should be an error
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michele.caini at gmail dot com
  Target Milestone: ---

Consider the following code:

    template<typename T>
    struct S { static constexpr int bar = T::foo; };

    struct U: S<U> { static constexpr int foo = 42; };

    int main() { }

GCC compiles it, but it shouldn't.
The type T is not a complete type when bar is initialized.

Here is a discussion on stackoverflow where reasons are discussed:
http://stackoverflow.com/questions/37816186/initializing-a-static-constexpr-data-member-of-the-base-class-by-using-a-static

Link to the standard (working draft) are provided as well in the discussion.

Other compilers (mostly clang) correctly reject it.