Is iterator to a container with incomplete data type legal?

Question

Is the following code legal?

class A
{
    std::map<int, A>::iterator x;
};

Visual Studio 2015 accepts it, but clang says

.../ndk/sources/cxx-stl/llvm-libc++/libcxx/include/utility:254:9:  
error: field has incomplete type 'A'
    _T2 second;
    ^
....
a.cpp:52:21:
note: definition of 'A' is not complete until the closing '}'
    struct A
           ^

Edit:
The problem seems to be with the standard library, http://rextester.com/QNNEG57036 fails on it

My question is whether the code is legal or not, not how to fix it (ex. by changing compiler flags).

Answer 1

Unless explicitly stated in the standard that incomplete types are legal, they are not legal. The specific section is 17.6.4.8 [res.on.functions] paragraph 2:

In particular, the effects are undefined in the following cases:

[...]

• if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.

I don't think any of containers is required to support incomplete types. Some of the smart pointers do allow incomplete types. Off-hand I can't think of anything else which would allow incomplete types. A quick search for "incomplete" yields the following components allowing incomplete types as template arguments:

• std::declval<T>()
• std::unique_ptr<T>
• std::default_delete<T>
• std::shared_ptr<T>
• std::weak_ptr<T>
• std::enable_shared_from_this<T>

In the code example, std::map<int, A>::iterator instantiates a template with an incomplete type. As a result the code results in undefined behavior.