c++ - Compile error when defining a member function, but only in GCC -

the following program compiles without errors msvs, clang , gcc:

class a;  namespace y {     using ::a;     class {}; }  int main() {} 

now let's define member function. still compiles msvs , clang, not gcc:

class a;  namespace y {     using ::a;     class { void f() {} }; }  int main() {} 

gcc gives following error message:

• prog.cc:5:22: error: definition of 'void a::f()' not in namespace enclosing 'a' [-fpermissive]

why that? bug in gcc?

if second version of program violates rule of c++ standard, rule violate , why doesn't msvs , clang give diagnostic message violation?

is case of ambiguity of c++ standard?

from error message looks gcc incorrectly thinks have violation of following rule:

• http://eel.is/c++draft/class.mfct#2 "a member function definition appears outside of class definition shall appear in namespace scope enclosing class definition."

we not have violation of rule since member function definition inside class definition. theory gcc confuses declaration class a; in global namespace class definition class { ... } in namespace y. think have bug in gcc.

with gcc declare same entity. can seen observing in first version of program possible use ::a complete type in main when compiling gcc. same msvs. clang declare different entities. difference may because of ambiguity in c++ standard. regardless of such ambiguity not violating http://eel.is/c++draft/class.mfct#2 . rule clear.

related question: class declaration in same scope using declaration compiles in gcc not msvs

both of these programs ill-formed according c++ standard. because of same reason in related question:

class declaration in same scope using declaration compiles in gcc not msvs

all compilers should give compile error in both cases: indicates bug in msvs, clang , gcc.

the bug in clang has been confirmed , fixed: https://llvm.org/bugs/show_bug.cgi?id=24030

the reason gcc gives strange error message second program gets confused when failed detect error present in both first , second program.