int main = 0, is compiled but crashed

I had very simple program as below.

int main = 0;

This program as a.c file compiled fine with gcc-4.8,gcc-5,gcc-6,gcc-7 and gcc-8 and application crashed when executed.

Same program as a.cpp file compiled fine with g++-4.8 and g++-5 and application crashed when executed. But with later version of g++ it gives compile time error.

Below is my questions.

1. Why a.cpp compile fine with g++-4.8 and 5, but not with later one? Does it bug in previous version of g++ OR some improvement in c++ standard?
2. If it compile successfully then why application get crashed?

Below is details compiler and output of compilation/execution.

+ gcc-4.8 --version
gcc-4.8 (Ubuntu 4.8.5-4ubuntu8) 4.8.5
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ gcc-4.8 c.c
+ ./a.out
Segmentation fault      (core dumped)
+ gcc-5 --version
gcc-5 (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ gcc-5 c.c
+ ./a.out
Segmentation fault      (core dumped)
+ gcc-6 --version
gcc-6 (Ubuntu 6.5.0-2ubuntu1~18.04) 6.5.0 20181026
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ gcc-6 c.c
+ ./a.out
Segmentation fault      (core dumped)
+ gcc-7 --version
gcc-7 (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ gcc-7 c.c
+ ./a.out
Segmentation fault      (core dumped)
+ gcc-8 --version
gcc-8 (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ gcc-8 c.c
+ ./a.out
Segmentation fault      (core dumped)
+ g++-4.8 --version
g++-4.8 (Ubuntu 4.8.5-4ubuntu8) 4.8.5
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ g++-4.8 c.cpp
+ ./a.out
Segmentation fault      (core dumped)
+ g++-5 --version
g++-5 (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ g++-5 c.cpp
+ ./a.out
Segmentation fault      (core dumped)
+ g++-6 --version
g++-6 (Ubuntu 6.5.0-2ubuntu1~18.04) 6.5.0 20181026
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ g++-6 c.cpp
c.cpp:1:5: error: cannot declare ‘::main’ to be a global variable
 int main = 0;
     ^~~~
+ g++-7 --version
g++-7 (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ g++-7 c.cpp
c.cpp:1:5: error: cannot declare ‘::main’ to be a global variable
 int main = 0;
     ^~~~
+ g++-8 --version
g++-8 (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

+ g++-8 c.cpp
c.cpp:1:5: error: cannot declare ‘::main’ to be a global variable
 int main = 0;
     ^~~~

Note: I already come across this but I doesn't answer my questions. Let me know if I need to add any specific tag to get attention of person with knowledge of this particular problem.