Program with extern decl-specifier in a class definition

Question

Consider the following program:

extern class A;

int main() {}

Is this well-formed according to the c++ standard? If it is ill-formed is diagnostics required? I'm getting different results for different compilers:

• Clang: No compiler errors (only a warning): http://melpon.org/wandbox/permlink/lhb8XNU01IyVhMnc
• GCC: Compiler error: http://melpon.org/wandbox/permlink/mIH9qmNY4noI1sEc
• Visual c++: No compiler errors (only a warning): http://webcompiler.cloudapp.net/

Answer 1

The program is ill-formed according to §7.1.1/1:

If a storage-class-specifier appears in a decl-specifier-seq, […] the init-declarator-list of the declaration shall not be empty (except for an anonymous union declared in a named namespace or in the global namespace, which shall be declared static (9.5)).