CWG2840 [basic.align] Missing requirements for fundamental alignments

[frederick-vs-ja]

Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [basic.align]

Link to reflector thread (if any):

Issue description: Given class definition struct B { long double d; }; (or any class definition in which alignas is not involved), these doesn't seem to be a rule forbidding B to have an extended alignment.

The issue was found and fixed in C (DR445).

Suggested resolution:

Change [basic.align] p2 as indicated:

A fundamental alignment is ~represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is~ a valid alignment less than or equal to equal to alignof(std​::​max_align_t) ([support.types]). Fundamental alignments shall be supported by the implementation for objects of all storage durations ([basic.stc]).The alignment required for a type may be different when it is used as the type of a complete object and when it is used as the type of a subobject. [Example 1: [...] — end example] The result of the alignof operator reflects the alignment requirement of the type in the complete-object case.

Change [basic.align] p3 as indicated:

An extended alignment is represented by an alignment greater than alignof(std​::​max_align_t). It is implementation-defined whether any extended alignments are supported and ~the contexts in~ the storage durations of objects for which they are supported ([dcl.align]). [...]

Add a note after [basic.align] p3 Note1:

[Note X: The strictest supported alignment can be different for objects with different storage durations. — end note]

Add a new paragraph after [basic.align] p3:

X. The alignment requirements of the following types are fundamental alignments:

• all basic types;
• all array types whose element type has a fundamental alignment requirement;
• all class types
  • none of whose non-static data members has a type with an extended alignment requirement,
  • none of whose non-static data members has an alignment-specifier ([dcl.align]) specifying an extended alignment, and
  • none of whose base classes has an extended alignment requirement.

Change [basic.align] p4 as indicated:

[...] Valid alignments include only ~those values returned by an alignof expression for the fundamental types~ fundamental alignments plus an additional implementation-defined set of values, which may be empty. [...]

Change [dcl.align] p2.2 as indicated:

if the constant expression does not evaluate to an alignment value ([basic.align]), or evaluates to an extended alignment and the implementation does not support that alignment ~in the context of the declaration~ for an object of the storage duration, if any, being declared, the program is ill-formed.

[jensmaurer]

CWG2840

[t3nsor]

I think the first bullet should actually say "a scalar type".

[jensmaurer]

Indeed. Fixed.