Is this valid c++ ?

Is this valid c++ ?

[Albert Hopkins]

I have the following code snippet:

typedef volatile struct {
} mystruct;

void mytest(mystruct* x) {};

As a C program (gcc) this compiles fine, but with g++ I get the
following error:

test.cpp:4: error: non-local function ‘void mytest(volatile mystruct*)’
uses anonymous type
test.cpp:2: error: ‘typedef volatile struct<anonymous> mystruct’ does
not refer to the unqualified type, so it is not used for linkage

So my question is: is g++ reporting the error incorrectly, or is it not
valid c++ and if so why?

Thanks in advance.