I can see a few bugs marked as SUSPENDED related to DR 253 and how there hadn't been an official resolution at the time. According to http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0490r0.html and http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253 it's part of C++17 (even though MSVC and clang implement it for older versions as well). IOW, I believe G++ should accept the following code: struct S { }; struct T { T() {} const S s_; }; rather than failing with const.cc: In constructor ‘T::T()’: const.cc:5:3: error: uninitialized const member in ‘const struct S’ [-fpermissive] T() {} ^ const.cc:6:11: note: ‘const S T::s_’ should be initialized const S s_; ^~
The resolution to the DR goes a bit further than the rule GCC has been using: A class type T is const-default-constructible if default-initialization of T would invoke a user-provided constructor of T (not inherited from a base class) or if - each direct non-variant non-static data member M of T has a default member initializer or, if M is of class type X (or array thereof), X is const-default-constructible, - if T is a union with at least one non-static data member, exactly one variant member has a default member initializer, - if T is a not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and - each potentially constructed base class of T is const-default-constructible.
I should note even though the other examples of DR 253 seems to be correctly accepting now; this one still fails. Note also EDG rejects this even though accepting the other examples of DR 253 issues; maybe EDG and GCC implement the same rules and not the expanded rules. Clang accepts it though.