This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1850. Differences between definition context and point of instantiation

[Moved to DR at the November, 2014 meeting.]

Various characteristics of entities referred to by a non-dependent reference in a template can change between the definition context and the point of instantiation of a specialization of that template. These include initialization (which affects whether an object can be used in a constant expression), function and template default arguments, and the completeness of types. There is implementation divergence as to whether these are checked in the definition context or at the point of instantiation. Presumably a rule is needed to make it ill-formed, no diagnostic required, if the validity of such a reference changes between the two contexts.

Proposed resolution (February, 2014):

Change 13.8 [temp.res] paragraph 8 as follows:

...If a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is done, and if the completeness of that type affects whether or not the program is well-formed or affects the semantics of the program, hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, the program is ill-formed; no diagnostic is required. If the interpretation of such a construct in the hypothetical instantiation is different from the interpretation of the corresponding construct in any actual instantiation of the template, the program is ill-formed; no diagnostic is required. [Note: This can happen in situations including the following:

• a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is performed, or

• an instantiation uses a default argument or default template argument that had not been defined at the point at which the template was defined, or

• constant expression evaluation (7.7 [expr.const]) within the template instantiation uses

• the value of a const object of integral or unscoped enumeration type or

• the value of a constexpr object or

• the value of a reference or

• the definition of a constexpr function,

and that entity was not defined when the template was defined, or

• a class template specialization or variable template specialization that is specified by a non-dependent simple-template-id is used by the template, and either it is instantiated from a partial specialization that was not defined when the template was defined or it names an explicit specialization that was not declared when the template was defined.

—end note] [Note: If a template is instantiated...