alignas and clang

[Thomas Stüfe]

Hi,

I'm not sure if this has been discussed before. With alignas, on clang, if
the natural type of the data has a larger alignment than what I requested
with alignas, I get a compile error if the specified alignment is smaller
than what would be the natural alignment of the type.

Example:

struct alignas(2) XX {
  void* p;
};

gives: error: requested alignment is less than minimum alignment of 8 for
type 'XX'

Happens on both MacOS and Linux clang build. Does not happen with GCC. Does
not happen with ATTRIBUTE_ALIGN(2).

Is this a clang bug? The standard [1] says:

"The object or the type declared by such a declaration will have its
alignment requirement equal to the strictest (largest) non-zero expression
of all alignas specifiers used in the declaration, unless it would weaken
the natural alignment of the type. "

That reads to me like gcc is correct.

This is a bit of a drawback compared to ATTRIBUTE_ALIGN, especially if
coding for different 32-bit and 64-bit platforms.

[1] https://en.cppreference.com/w/cpp/language/alignas
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