Why does default-constructibility behave weirdly for inner structs with NSDMI?

Consider the following code:

#include <type_traits>

struct outer {
    struct inner {
        unsigned int x = 0;
    };
//    static_assert(std::is_default_constructible<inner>::value,
//          "not default ctorable - inside");
};

static_assert(std::is_default_constructible<outer::inner>::value,
    "not default ctorable - outside");

This compiles fine. But - if I uncomment the static assert inside outer - both asserts fail with clang++ and gcc++. Why should they not both pass?

Notes:

• If you remove the initializer of x and enable the assertions, the code compiles.

• This:

  #include <type_traits>
  
  struct outer {
       struct inner {
           unsigned int x = 0;
       };
  
       inner get_an_inner() { 
           static_assert(std::is_default_constructible<outer::inner>::value,
               "not default ctorable - outside");
           return inner{}; 
       }
  };
  

  compiles!

• There's a LLVM bug report about basically the same thing, but it's not just clang and the bug report has not gotten any comments.