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| 1 | +<?xml version='1.0' encoding='utf-8' standalone='no'?> |
| 2 | +<!DOCTYPE issue SYSTEM "lwg-issue.dtd"> |
| 3 | + |
| 4 | +<issue num="4326" status="New"> |
| 5 | +<title>Standard defined queries can't customize `forwarding_query_t` by inheriting from it</title> |
| 6 | +<section> |
| 7 | +<sref ref="[exec.fwd.env]"/> |
| 8 | +</section> |
| 9 | +<submitter>Lewis Baker</submitter> |
| 10 | +<date>25 Aug 2025</date> |
| 11 | +<priority>99</priority> |
| 12 | + |
| 13 | +<discussion> |
| 14 | +<p> |
| 15 | +The definition of the `forwarding_query_t` (<sref ref="[exec.fwd.env]"/>) states that a query |
| 16 | +is a forwarding-query if either the expression `q.query(forwarding_query)` is a constant expression |
| 17 | +that evaluates to `true` or the query type inherits from `forwarding_query_t`. |
| 18 | +<p/> |
| 19 | +However, all of the queries defined in <sref ref="[exec]"/> are defined similar to the following: |
| 20 | +</p> |
| 21 | +<blockquote><pre> |
| 22 | +struct get_allocator_t { <i>unspecified</i> }; |
| 23 | +</pre></blockquote> |
| 24 | +<p> |
| 25 | +While this definition allows for the members of the class to be unspecified, it still explicitly |
| 26 | +specifies that it has an empty list of base-classes and therefore cannot customize |
| 27 | +`forwarding_query_t` by inheriting from `forwarding_query_t`. |
| 28 | +<p/> |
| 29 | +We should consider whether we want to change the specification of queries that are to be |
| 30 | +forwarding-queries to allow inheritance from `forwarding_query_t` or otherwise remove the |
| 31 | +support for customizing `forwarding_query_t` by inheriting from it. |
| 32 | +<p/> |
| 33 | +<b>Proposed Approach:</b> |
| 34 | +<p/> |
| 35 | +Remove the rule that checks for inheritance from `forwarding_query_t`. Any query can already |
| 36 | +customize `forwarding_query_t` by defining a `query(forwarding_query_t)` member function, so |
| 37 | +there is no loss of functionality from doing this and it would simplify the design. |
| 38 | +</p> |
| 39 | +</discussion> |
| 40 | + |
| 41 | +<resolution> |
| 42 | +<p> |
| 43 | +This wording is relative to <paper num="N5014"/>. |
| 44 | +</p> |
| 45 | + |
| 46 | +<ol> |
| 47 | + |
| 48 | +<li><p>Modify <sref ref="[exec.fwd.env]"/> as indicated:</p> |
| 49 | + |
| 50 | +<blockquote> |
| 51 | +<p> |
| 52 | +-2- The name `forwarding_query` denotes a query object. For some query object `q` of type `Q`, |
| 53 | +`forwarding_query(q)` is expression-equivalent to: |
| 54 | +</p> |
| 55 | +<ol style="list-style-type: none"> |
| 56 | +<li><p>(2.1) — <tt><i>MANDATE-NOTHROW</i>(q.query(forwarding_query))</tt> if that expression is well-formed. |
| 57 | +<p/> |
| 58 | +<i>Mandates</i>: The expression above has type `bool` and is a core constant expression if `q` is a core constant |
| 59 | +expression. |
| 60 | +</p></li> |
| 61 | +<li><p><del>(2.2) — Otherwise, `true` if <tt>derived_from<Q, forwarding_query_t></tt> is `true`.</del></p></li> |
| 62 | +<li><p>(2.3) — Otherwise, `false`.</p></li> |
| 63 | +</ol> |
| 64 | + |
| 65 | +</blockquote> |
| 66 | + |
| 67 | +</li> |
| 68 | + |
| 69 | +</ol> |
| 70 | +</resolution> |
| 71 | + |
| 72 | +</issue> |
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