Attributes for the result name in a postcondition assertion

• Introduction
• Motivation
• Proposal
• Implementation experience
• Acknowledgements
• References
• Wording

Introduction

SG21 confirmed support for P3088R1 (Attributes for contract assertions) ^[P3088R1] during its 2024-02-15 telecon and the approved changes were subsequently added to P2900R5 (Contracts for C++) ^[P2900R5]. This established syntactic locations for attributes that appertain to contract assertions and the assertion statement as follows:

int f(int i, int j) pre [[foo]] (i > 0 && j > 0) post [[bar]] (r: r > 0) { [[likely]] contract_assert [[baz]] (i + j < 10); return i + j; }

int g() post (r [[maybe_unused]]: r > 0);

Motivation

As evidenced by annex A (Grammar summary) in the C++ standard, an attribute may appertain to almost every kind of declared name or entity including:

• Type aliases (when declared with an alias-declaration)
• Enumerations
• Enumerators
• Variables (including the unnamed object for structured bindings)
• Structured bindings (if P0609R2 ^[P0609R2] is adopted)
• Classes
• Data members
• Functions
• Parameters (function parameters, non-type template parameters, exception handlers)
• Concepts
• Labels
• Namespaces
• Modules

• Namespace aliases
• Type template parameters
• Template template parameters
• Lambda captures

Recommended practice: Implementations should use the deprecated attribute to produce a diagnostic message in case the program refers to a name or entity other than to declare it, after a declaration that specifies the attribute. The diagnostic message should include the text provided within the attribute-argument-clause of any deprecated attribute applied to the name or entity. The value of a has-attribute-expression for the deprecated attribute should be 0 unless the implementation can issue such diagnostic messages.

Recommended practice: For an entity marked maybe_unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused. For a structured binding declaration not marked maybe_unused, implementations should not emit such a warning unless all of its structured bindings are unused. For a label to which maybe_unused is applied, implementations should not emit a warning that the label is used or unused. The value of a has-attribute-expression for the maybe_unused attribute should be 0 if the attribute does not cause suppression of such warnings.

The [[deprecated]] attribute was adopted for C++14 via N3760 ^[N3760]. Neither that paper, its previous revision, or the minutes from EWG and CWG review during the following meetings demonstrate consideration for use of the attribute with a namespace alias.

• EWG review of N3394 in Portland, 2012
• CWG review of N3760 in Chicago, 2013

The [[maybe_unused]] attribute was adopted for C++17 via P0212R1 ^[P0212R1]. Neither that paper, its previous revision, the rationale provided in the previous P0068R0 ^[P0068R0], or the minutes from EWG and CWG review during the following meetings demonstrate consideration for use of the attribute with a type template parameter, template template parameter, or lambda capture.

• EWG review of P0068R0 in Kona, 2015
• CWG review of D0212R1 in Jacksonville, 2016

A search of both active and closed CWG issues failed to identify any existing issues that concern attribute appertainment for the excluded items.

There does not appear to be a recorded rationale for omitting attribute support for the excluded items above.

Absent compelling rationale otherwise, precedent indicates that new kinds of declared names or entities should allow for attribute appertainment and their respective decarations should therefore include syntactic locations for an attribute specifier sequence.

Proposal

• Allow attribute-specifier-seq to optionally follow identifier in a result-name-introducer with the effect that the specified attributes (if any) appertain to the declared result name.
• Allow the [[maybe_unused]] attribute to appertain to a result name.

Implementation experience

None. The experimental implementation of P2900R5 ^[P2900R5] in gcc hosted on https://godbolt.org does not currently diagnose an unused result name, nor does it allow an attribute specifier sequence to appear in a result-name-introducer as demonstrated at https://godbolt.org/z/339WEr7c3.

Acknowledgements

Thanks to Timur Doumler and Joshua Berne for their previous work to enable attributes for contract assertions in P3088R1 ^[P3088R1] and for the tremendous amount of time, energy, and expertise they have each contributed to SG21 and advancing contracts for C++.

Additional thanks to Timur Doumler for suggesting much of the wording included in this proposal.

References

Wording

These changes are relative to P2900R5 ^[P2900R5] and P0609R2 ^[P0609R2].

Modify [dcl.contract.func]:
Drafting note: This depends on the addition of attributed-identifier from P0609R2 ^[P0609R2].

function-contract-specifier-seq :

function-contract-specifier function-contract-specifier-seq

function-contract-specifier :

precondition-specifier postcondition-specifier

precondition-specifier :

pre attribute-specifier-seq_opt ( conditional-expression )

postcondition-specifier :

post attribute-specifier-seq_opt ( result-name-introducer_opt conditional-expression )

result-name-introducer :

attributed-identifier :

A function contract assertion is a contract assertion ([basic.contract]) associated with a function. Each function-contract-specifier of a function-contract-specifier-seq (if any) of an unspecified first declaration of a function introduces a corresponding function contract assertion for that function. The optional attribute-specifier-seq following pre or post appertains to the introduced contract assertion. The optional attribute-specifier-seq of the attributed-identifier in a result-name-introducer appertains to the introduced result name. [ Note: The function-contract-specifier-seq of a lambda-declarator applies to the call operator or operator template of the corresponding closure type ([expr.prim.lambda.closure]). — end note ]

[ … ]

Add a proposed modification to [dcl.attr.unused], paragraph 2:
Drafting note: This includes modifications from P0609R2 ^[P0609R2].

The attribute may be applied to the declaration of a class, typedef-name, variable (including a structured binding declaration), structured binding, result name, non-static data member, function, enumeration, or enumerator, or to an identifier label ([stmt.label]).