Allow __restrict'ing in array size specification in C++, like C

[eyalroz]

Unfortunately, C++ does not have the ever-so-useful restrict keyword of C. Fortunately, compilers such as clang++ offer a __restrict pseudo-keyword. Unfortunately, that keyword is only offered for pointers, not for array parameters. If we write:

int foo(int a[__restrict 10], int b[__restrict 10]) {
    a[0] += b[0];
    return a[0] + b[0];
}

int bar(int *a, int *b) {
    a[0] += b[0];
    return a[0] + b[0];
}

int baz(int * __restrict a, int * __restrict b) {
    a[0] += b[0];
    return a[0] + b[0];
}

then baz() will be accepted by clang++ as C++, but foo() will not (GodBolt).

Well, foo() should be accepted! Taking array parameters is useful, especially if they are multi-dimensionsional. Without them,

See also this G++ bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97477

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eyal Rozenberg (eyalroz)

Unfortunately, C++ does not have the ever-so-useful `restrict` keyword of C. Fortunately, compilers such as clang++ offer a `__restrict` pseudo-keyword. Unfortunately, that keyword is only offered for pointers, not for array parameters. If we write: ``` int foo(int a[__restrict 10], int b[__restrict 10]) { a[0] += b[0]; return a[0] + b[0]; }

int bar(int *a, int *b) {
a[0] += b[0];
return a[0] + b[0];
}

int baz(int * __restrict a, int * __restrict b) {
a[0] += b[0];
return a[0] + b[0];
}

then `baz()` will be accepted by clang++ as C++, but `foo()` [will not](https://godbolt.org/z/sK7eWGG9j) (GodBolt).

Well, `foo()` should be accepted! Taking array parameters is useful, especially if they are multi-dimensionsional. Without them,

See also this G++ bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97477


  [1]: https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html
</details>

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Eyal Rozenberg (eyalroz)

Unfortunately, C++ does not have the ever-so-useful `restrict` keyword of C. Fortunately, compilers such as clang++ offer a `__restrict` pseudo-keyword. Unfortunately, that keyword is only offered for pointers, not for array parameters. If we write: ``` int foo(int a[__restrict 10], int b[__restrict 10]) { a[0] += b[0]; return a[0] + b[0]; }

int bar(int *a, int *b) {
a[0] += b[0];
return a[0] + b[0];
}

int baz(int * __restrict a, int * __restrict b) {
a[0] += b[0];
return a[0] + b[0];
}

then `baz()` will be accepted by clang++ as C++, but `foo()` [will not](https://godbolt.org/z/sK7eWGG9j) (GodBolt).

Well, `foo()` should be accepted! Taking array parameters is useful, especially if they are multi-dimensionsional. Without them,

See also this G++ bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97477


  [1]: https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html
</details>