| Document number: | P0520R0 |
| Date: | 2016-11-11 |
| Project: | Programming Language C++ |
| Reference: | ISO/IEC IS 14882:2014 |
| Reply to: | William M. Miller |
| Edison Design Group, Inc. | |
| wmm@edg.com |
Section references in this document reflect the section numbering of document WG21 N4582.
The resolution of issue 496 included the addition of 12.8 [class.copy] paragraph 25.2, making a class's copy/move constructor non-trivial if it has a non-static data member of volatile-qualified type. This change breaks the IA-64 ABI, so it has been requested that CWG reconsider this aspect of the resolution.
On a related note, the resolution of issue 496 also changed 3.9 [basic.types] paragraph 9, which makes volatile-qualified scalar types “trivial” but not “trivially copyable.” It is not clear why there is a distinction made here; the only actual use of “trivial type” in the Standard appears to be in the description of qsort, which should probably use “trivially copyable.” (See also issue 1746.)
Notes from the February, 2016 meeting:
CWG agreed with the suggested direction for the changes in 12.8 [class.copy]; the use of “trivial” will be dealt with separately and not as part of the resolution of this issue.
Proposed resolution (June, 2016):
Change 3.9 [basic.types] paragraph 9 as follows:
...called POD types. Cv-unqualified scalar types, trivially copyable class types (Clause 9 [class]), arrays of such types, and non-volatile const-qualified cv-qualified versions of these types (3.9.3 [basic.type.qualifier]) are collectively called trivially copyable types. Scalar types...
Delete bullet 12.2 of 12.8 [class.copy]:
A copy/move constructor for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if
...
class X has no non-static data members of volatile-qualified type, and
...
Delete bullet 25.2 of 12.8 [class.copy]:
A copy/move assignment operator for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if
...
class X has no non-static data members of volatile-qualified type, and
According to bullet 2.4 of 14.6.2.3 [temp.dep.constexpr], an id-expression is value-dependent if
it names a static data member that is a dependent member of the current instantiation and is not initialized in a member-declarator,
This implies that the address of an initialized static data member is not value-dependent, which is incorrect.
Proposed resolution (June, 2016):
Change 14.6.2.3 [temp.dep.constexpr] paragraph 5 as follows:
An expression of the form &qualified-id where the qualified-id names a dependent member of the current instantiation is value-dependent. An expression of the form
&cast-expression
is also value-dependent if evaluating cast-expression as a core constant expression (5.20 [expr.const]) succeeds and the result of the evaluation refers to a templated entity that is an object with static or thread storage duration or a member function.