http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59123
Bug ID: 59123 Summary: [c++11] can't forward-declare an object later defined constexpr Product: gcc Version: 4.8.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: eric.niebler at gmail dot com Clang accepts the following. I think it's legal. g++ 4.8.1 rejects it: // Fwd-declarations struct S; extern const S s; // (... later) definitions struct S {}; constexpr S s {};