is a c++ array of size N a distinct type

I thought that

int a1[5];

and

int a2[6];

were distinct types.

But

void foo(int a[5]){};
void foo(int a[6]){};

will not compile saying that they are duplicated definitions of foo (ie foo(int *a))

I was very surprised by this. One of the reasons I saw given for why VLAs are not allowed in the c++ standard is because they break the type system.

I was also surprised to find c doing the same thing.

void foo(int a[4]){
   printf("sz=%ld", sizeof(a));
}

reports a pointer size. I expected it to report 4*sizeof(int) (and only accept a variable of type int[4] to be accepted as a call argument)

>Solution :

Yes, they are distinct types, this can be confirmed with:

int a[5];
int b[6];
static_assert(!std::is_same_v<decltype(a), decltype(b)>);

Your problem is that in function parameter lists, arrays are silently replaced with pointers, so both void foo(int a[5]) and void foo(int a[6]) end up as void foo(int *a).

This only happens to arrays, as opposed to e.g. references to arrays, so void foo(int (&a)[5]) would work (or int (*a)[5] in C).