The difference in constant between gcc and clang

Question

What are the various error reports between gcc (read-only assignment) and clang (no errors) when compiling the same code?

Consider the following C code snippet:

#include <stdio.h>

struct ex {
    const int x;
    int y;
};

int main(void)
{
    struct ex a;
    const struct ex b = {.x = 44, .y = 66};

    a = b;

    printf("%d, %d\n", a.x, a.y);
    printf("%d, %d\n", b.x, b.y);

    return 0;
}

NCA

- version

gcc (x86_64-posix-seh-rev1, Built by MinGW-W64 project) 6.2.0

gcc -Wall -Wextra -pedantic -std = c99 test.c

test.c: In function 'main':
test.c:11:7: error: assignment of read-only variable 'a'
     a = b;
       ^

clank

- version

clang version 3.9.1 (branches/release_39)
Target: x86_64-pc-windows-msvc
Thread model: posix

clang -Weverything -std = c99 test.c

<no errors>