gcc-10 (10.3.0) does not handle anonymous struct correctly?

$ gcc-10 --version
gcc-10 (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0

$ cat test.c
struct A {
    int a;
};

struct B {
    int b;
    struct A;
};

int main(void)
{
    struct B c;
    c.a = 0;
    c.b = 1;
    return 0;
}

$ gcc-10 test.c
test.c:7:13: warning: declaration does not declare anything
    7 |     struct A;
      |             ^
test.c: In function ‘main’:
test.c:13:6: error: ‘struct B’ has no member named ‘a’
   13 |     c.a = 0;
      |      ^

Adding -std=c17 does not make any difference. I checked (with the -v option that cc1 gets called from the correct location (/usr/lib/gcc/x86_64-linux-gnu/10/cc1 ). The source should compile, though. gcc 10.2.0 (used on Cygwin, for example), is perfectly able to deal with this code:

$ gcc --version
gcc (GCC) 10.2.0
$ gcc test.c
$

(no errors).

This is what was installed as "gcc-10":

cc-10-base/focal-updates,focal-security,now 10.3.0-1ubuntu1~20.04 amd64 [installed,automatic]
gcc-10/focal-updates,focal-security,now 10.3.0-1ubuntu1~20.04 amd64 [installed]
libgcc-10-dev/focal-updates,focal-security,now 10.3.0-1ubuntu1~20.04 amd64 [installed,automatic]

My system:

$ uname -a
Linux xxx 5.11.0-25-generic #27~20.04.1-Ubuntu SMP Tue Jul 13 17:41:23 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Any help will be greatly appreciated!

Thanks!

Looks like -fms-extensions is implicitly defined under Cygwin, so adding it explicitly to the gcc command on Ubuntu solves the problem.