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Section: 3.41 [defns.prog.def.spec] Status: C++20 Submitter: Hubert Tong Opened: 2018-06-09 Last modified: 2021-06-06
Priority: 2
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Discussion:
The description of closure types in 7.5.6.2 [expr.prim.lambda.closure] says:
An implementation may define the closure type differently […]
The proposed resolution to LWG 2139(i) defines a "program-defined type" to be a
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization
I am not sure that the intent of whether closure types are or are not program-defined types is clearly conveyed by the wording.
[2018-06-23 after reflector discussion]
Priority set to 2
[2018-08-14 Casey provides additional discussion and a Proposed Resolution]
We use the term "program-defined" in the library specification to ensure that
two users cannot create conflicts in a component in namespace std by
specifying different behaviors for the same type. For example, we allow users to
specialize common_type when at least one of the parameters is a
program-defined type. Since two users cannot define the same program-defined
type, this rule prevents two users (or libraries) defining the same
specialization of std::common_type.
Since it's guaranteed that even distinct utterances of identical lambda expressions produce closures with distinct types (7.5.6.2 [expr.prim.lambda.closure]), adding closure types to our term "program-defined type" is consistent with the intended use despite that such types are technically defined by the implementation.
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 3.42 [defns.prog.def.type] as follows:
program-defined type
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation (except for closure types (7.5.6.2 [expr.prim.lambda.closure]) for program-defined lambda expressions), or an instantiation of a program-defined specialization
[2018-08-23 Batavia Issues processing]
Updated wording
[2018-11 San Diego Thursday night issue processing]
Status to Ready.
Proposed resolution:
This wording is relative to N4762.
Modify 3.42 [defns.prog.def.type] as follows:
program-defined type
non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, or an instantiation of a program-defined specialization