[clang++] Does not error with a zero length array declaration (definition)

[safocl]

Clang++ allows declaration (definition) of arrays of zero size (TYPE NAME[0]). However, this is prohibited by the c++ standard:
[c++23] (here)

In a declaration T D where D has the form
D1 [ constant-expression opt ] attribute-specifier-seq opt
[...]
The constant-expression shall be a converted constant expression of type std​::​size_t ([expr.const]).
Its value N specifies the array bound, i.e., the number of elements in the array; N shall be greater than zero.

[c++11] (here)

[c++17] (here)

Example :

#include <iostream>

struct Data {
   int lhs[0];
   explicit Data(const int rhs) {
    lhs[rhs];
    lhs[0]=rhs;
   }

   void print() {
      std::cout << lhs[0] << "\n";
   }
};


int main() {
   int aaa[0];
   int bbb[0]{};
   Data t(42);
   t.print();
   return sizeof(aaa) + sizeof(bbb);
}

Expected behavior
COMPILETIME ERROR.

Current behavior
Compilation completed without errors.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (safocl)

Clang++ allows declaration (definition) of arrays of zero size (`TYPE NAME[0]`). However, this is prohibited by the c++ standard: [c++23] ([here](https://timsong-cpp.github.io/cppwp/n4950/dcl.array#1.sentence-3)) ``` In a declaration T D where D has the form D1 [ constant-expression opt ] attribute-specifier-seq opt [...] The constant-expression shall be a converted constant expression of type std​::​size_t ([expr.const]). Its value N specifies the array bound, i.e., the number of elements in the array; N shall be greater than zero. ```

[c++11] ([here](https://timsong-cpp.github.io/cppwp/n3337/dcl.array#1))

[c++17] ([here](https://timsong-cpp.github.io/cppwp/n4659/dcl.array#1))

Example :

#include <iostream>

struct Data {
   int lhs[0];
   explicit Data(const int rhs) {
    lhs[rhs];
    lhs[0]=rhs;
   }

   void print() {
      std::cout << lhs[0] << "\n";
   }
};


int main() {
   int aaa[0];
   int bbb[0]{};
   Data t(42);
   t.print();
   return sizeof(aaa) + sizeof(bbb);
}

Expected behavior
COMPILETIME ERROR.

Current behavior
compilation is done without error.

[MacDue]

Note: If you use -pedantic -Werror this does become an error:

<source>:4:12: error: zero size arrays are an extension [-Werror,-Wzero-length-array]
    4 |    int lhs[0];
      |            ^

https://godbolt.org/z/717Y6xn3c

[safocl]

Note: If you use -pedantic -Werror this does become an error:

<source>:4:12: error: zero size arrays are an extension [-Werror,-Wzero-length-array]
    4 |    int lhs[0];
      |            ^

This error is a limitation of the core language - it should work without additional compiler options (which only help to generate correct code).
(this is not semantics, but syntax -- such a code entry is unacceptable in principle)

https://godbolt.org/z/1TfPEcr1f -- it does error for the The constant-expression shall be a converted constant expression of type std​::​size_t ([expr.const]) rule, that have same rank with issue

<source>:17:12: error: value of type 'const char[1]' is not implicitly convertible to 'unsigned long'
   17 |    int aaa[""];
      |            ^~

[shafik]

This is a GNU extension: #54705

There is some question as to why this allowed outside of a flexible array context but I am not sure how much appetite there would be to change this as we would need to coordinate with both gcc/edg.

I am going to close this but perhaps in the future someone might want to consider thinking about this.