The reason for not compiling safeD code on the gdc compiler

[Heikuw]

What causes this example of safeD code to be compiled in dmd and 
ldc but not compiled in gcc?

code :

     immutable(int)* f(int* p) @trusted
     {
         version (none) p[2] = 13;
         // Invalid. p[2] is out of bounds. This line would 
exhibit undefined
         // behavior.

         version (none) p[1] = 13;
         // Invalid. In this program, p[1] happens to be 
in-bounds, so the
         // line would not exhibit undefined behavior, but a 
trusted function
         // is not allowed to rely on this.

         version (none) return cast(immutable) p;
         // Invalid. @safe code still has mutable access and could 
trigger
         // undefined behavior by overwriting the value later on.

         int* p2 = new int;
         *p2 = 42;
         return cast(immutable) p2;
         // Valid. After f returns, no mutable aliases of p2 can 
exist.
     }

     void main() @safe
     {
         int[2] a = [10, 20];
         int* mp = &a[0];
         immutable(int)* ip = f(mp);
         assert(a[1] == 20); // Guaranteed. f cannot access a[1].
         assert(ip !is mp); // Guaranteed. f cannot introduce 
unsafe aliasing.
     }


     result code in gdc compiler :
     main.d:25:15: error: cannot take address of local a in @safe 
function main
        25 |     int* mp = &a[0];
           |               ^