CWG3024 [dcl.align] Unclear effect of alignment specifier on a reference #703

Full name of submitter: Brian Bi

Issue description: [dcl.align]/1 permits alignment specifiers to be applied to references. p4 then specifies that an alignment specifier affects the "alignment requirement" of the entity, but this is meaningless for a reference.

There is implementation divergence, with GCC and MSVC permitting an alignment specifier with any value for a reference, while Clang and EDG treat the natural alignment of a reference as being equal to that of a pointer and disallow weaker alignment specifiers. However, note that it is not possible to query the alignment of a reference, since alignof accepts only types.

Suggested resolution: Edit [dcl.align]/1 as follows, with a drive-by fix for the fact that static data members are already variables:

An alignment-specifier may be applied to a variable or to a classnon-static data member, but it shall not be applied to a reference, a bit-field, a function parameter, or an exception-declaration ([except.handle]). [...]

Alternatively, we could allow it, leaving [dcl.align]/1 alone and perhaps edit [dcl.align]/4:

The effect of an alignment-specifier applied to a reference is implementation-defined. Otherwise, the The alignment requirement of an entity is the strictest nonzero alignment specified by its alignment-specifiers, if any; otherwise, the alignment-specifiers have no effect.