P2552R3 On the ignorability of standard attributes

Author: jensmaurer

source/declarations.tex

@@ -8697,6 +8697,11 @@
 deduce information used to optimize the program.
 Implementations are not required to deduce
 any information from any particular assumption.
+It is expected that the value of
+a \grammarterm{has-attribute-expression} for the \tcode{assume} attribute
+is \tcode{0}
+if an implementation does not attempt to deduce
+any such information from assumptions.
 \end{note}
 \begin{example}
 \begin{codeblock}
@@ -8845,6 +8850,10 @@
 declaration that specifies the attribute. The diagnostic message should include the text provided
 within the \grammarterm{attribute-argument-clause} of any \tcode{deprecated} attribute applied
 to the name or entity.
+The value of
+a \grammarterm{has-attribute-expression} for the \tcode{deprecated} attribute
+should be \tcode{0}
+unless the implementation can issue such diagnostic messages.
 
 \rSec2[dcl.attr.fallthrough]{Fallthrough attribute}
 \indextext{attribute!fallthrough}
@@ -8871,6 +8880,10 @@
 a warning that an implementation might otherwise issue
 for a case or default label that is reachable
 from another case or default label along some path of execution.
+The value of
+a \grammarterm{has-attribute-expression} for the \tcode{fallthrough} attribute
+should be \tcode{0}
+if the attribute does not cause suppression of such warnings.
 Implementations should issue a warning
 if a fallthrough statement is not dynamically reachable.
 
@@ -8920,7 +8933,7 @@
 that contains the \grammarterm{attribute-token} \tcode{unlikely}.
 
 \pnum
-\recommended
+\begin{note}
 The use of the \tcode{likely} attribute
 is intended to allow implementations to optimize for
 the case where paths of execution including it
@@ -8933,8 +8946,14 @@
 are arbitrarily more unlikely
 than any alternative path of execution
 that does not include such an attribute on a statement or label.
+It is expected that the value of a \grammarterm{has-attribute-expression}
+for the \tcode{likely} and \tcode{unlikely} attributes
+is \tcode{0}
+if the implementation does not attempt to use these attributes
+for such optimizations.
 A path of execution includes a label
 if and only if it contains a jump to that label.
+\end{note}
 \begin{note}
 Excessive usage of either of these attributes
 is liable to result in performance degradation.
@@ -8995,6 +9014,10 @@
 For a structured binding declaration not marked \tcode{maybe_unused},
 implementations should not emit such a warning unless
 all of its structured bindings are unused.
+The value of
+a \grammarterm{has-attribute-expression} for the \tcode{maybe_unused} attribute
+should be \tcode{0}
+if the attribute does not cause suppression of such warnings.
 
 \pnum
 \begin{example}
@@ -9063,6 +9086,9 @@
 a potentially-evaluated discarded-value expression\iref{expr.prop}
 is discouraged unless explicitly cast to \keyword{void}.
 Implementations should issue a warning in such cases.
+The value of
+a \grammarterm{has-attribute-expression} for the \tcode{nodiscard} attribute
+should be \tcode{0} unless the implementation can issue such warnings.
 \begin{note}
 This is typically because discarding the return value
 of a nodiscard call has surprising consequences.
@@ -9125,6 +9151,9 @@
 \recommended
 Implementations should issue a
 warning if a function marked \tcode{[[noreturn]]} might return.
+The value of
+a \grammarterm{has-attribute-expression} for the \tcode{noreturn} attribute
+should be \tcode{0} unless the implementation can issue such warnings.
 
 \pnum
 \begin{example}
@@ -9159,6 +9188,14 @@
 at the end of the object
 can be reused as storage for other members.
 \end{note}
+
+\recommended
+The value of a \grammarterm{has-attribute-expression}
+for the \tcode{no_unique_address} attribute
+should be \tcode{0} for a given implementation
+unless this attribute can cause a potentially-overlapping subobject
+to have zero size.
+
 \begin{example}
 \begin{codeblock}
 template<typename Key, typename Value,

source/preprocessor.tex

@@ -375,8 +375,10 @@
 
 \pnum
 For an attribute specified in this document,
-the value of the \grammarterm{has-attribute-expression}
-is given by \tref{cpp.cond.ha}.
+it is \impldef{value of \grammarterm{has-attribute-expression}
+for standard attributes}
+whether the value of the \grammarterm{has-attribute-expression}
+is \tcode{0} or is given by \tref{cpp.cond.ha}.
 For other attributes recognized by the implementation,
 the value is
 \impldef{value of \grammarterm{has-attribute-expression}