Clang rejects valid int iarr [10] alignas(Required);

[ranaanoop]

The following valid program is rejected by clang but accepted by both gcc and msvc. Demo

constexpr std::size_t Required=16;

int iarr [10] alignas(Required); 

Clang says:

<source>:16:15: error: 'alignas' attribute cannot be applied to types
   16 | int iarr [10] alignas(Required); 
      |          

Note that if you move [10] to after alignas(Required) then clang also starts accepting.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Anoop Rana (ranaanoop)

The following valid program is rejected by clang but accepted by both gcc and msvc. [Demo](https://godbolt.org/z/j54c4KxMa)

constexpr std::size_t Required=16;

int iarr [10] alignas(Required); 

Clang says:

<source>:16:15: error: 'alignas' attribute cannot be applied to types
   16 | int iarr [10] alignas(Required); 
      |          

Note that if you move [10] to after alignas(Required) then clang also starts accepting.

[MitalAshok]

GCC/MSVC also accept int iarr [10] alignas(16) [14];

My understanding is that in that position, alignas appertains to the array type and this should be invalid.

https://eel.is/c++draft/dcl.attr.grammar#5.sentence-1

https://eel.is/c++draft/dcl.dcl#ntref:attribute-specifier-seq______________________

Clang does accept int iarr2 [10] __attribute__((aligned(16)));, as described here https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#All-other-attributes:

For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. [...]. If an attribute that only applies to declarations is applied to the type of a declaration, it is treated as applying to that declaration

But does not accept int iarr3 [10] [[gnu::aligned(16)]]; like gcc does