Attributes for the result name in a postcondition assertion


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; }
However, P3088R1 did not provide for an attribute to appertain to the result name optionally declared in a postcondition specifier; the r declared in the example postcondition above. This proposal seeks to fill that gap by allowing an attribute to be specified in the result name introducer.
int g() post (r [[maybe_unused]]: r > 0);


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:

Exceptions include: It is not clear why attributes are not supported for the items in the exceptions list. The recommended practice for the [[deprecated]] attribute in [dcl.attr.deprecated]p4 seems applicable to namespace aliases:
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.
Likewise, the recommended practice for the [[maybe_unused]] attribute in [dcl.attr.unused]p4 seems applicable to the other entities:
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.

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.

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.


Implementation experience

None. The experimental implementation of P2900R5 [P2900R5] in gcc hosted on 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


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.


[N3760] "[[deprecated]] attribute", N3760, 2013.
[P0068R0] "Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes.", P0068R0, 2015.
[P0212R1] "Wording for [[maybe_unused]] attribute.", P0212R1, 2016.
[P0609R2] "Attributes for Structured Bindings", P0609R2, 2023.
[P2900R5] "Contracts for C++", P2900R5, 2024.
[P3088R1] "Attributes for contract assertions", P3088R1, 2024.


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

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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-seqopt ( conditional-expression )

postcondition-specifier :
post attribute-specifier-seqopt ( result-name-introduceropt 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]).