[P2996] Rejecting size_of(bitfield)

Author: brevzin

2996_reflection/d2996r12.html

@@ -1090,6 +1090,9 @@ <h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history
 <ul>
 <li>improve specification of <code class="sourceCode cpp">access_context<span class="op">::</span>current<span class="op">()</span></code>
 (including examples)</li>
+<li><code class="sourceCode cpp">size_of<span class="op">(</span>r<span class="op">)</span></code>
+is no longer constant if <code class="sourceCode cpp">r</code> is a
+bit-field</li>
 </ul></li>
 </ul>
 <p>Since <span class="citation" data-cites="P2996R10"><a href="https://wg21.link/p2996r10" role="doc-biblioref">[P2996R10]</a></span>:</p>
@@ -13473,8 +13476,14 @@ <h3 class="unnumbered" id="meta.reflection.layout-reflection-layout-queries">[me
 <p><span class="marginalizedparent"><a class="marginalized" href="#pnum_629" id="pnum_629">5</a></span>
 <em>Constant When</em>: <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code>
 is a reflection of a type, object, value, variable of non-reference
-type, non-static data member, direct base class relationship, or data
-member description. If <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code>
+type, non-static data member that is not a bit-field, direct base class
+relationship, or data member description
+(<code class="sourceCode cpp"><em>T</em></code>,
+<code class="sourceCode cpp"><em>N</em></code>,
+<code class="sourceCode cpp"><em>A</em></code>,
+<code class="sourceCode cpp"><em>W</em></code>,
+<code class="sourceCode cpp"><em>NUA</em></code>) ([class.mem.general])
+where <code class="sourceCode cpp"><em>W</em></code> is not ⊥. If <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code>
 represents a type, then <code class="sourceCode cpp">is_complete_type<span class="op">(</span>r<span class="op">)</span></code>
 is <code class="sourceCode cpp"><span class="kw">true</span></code>.</p>
 <p><span class="marginalizedparent"><a class="marginalized" href="#pnum_630" id="pnum_630">6</a></span>
@@ -13485,8 +13494,7 @@ <h3 class="unnumbered" id="meta.reflection.layout-reflection-layout-queries">[me
 <code class="sourceCode cpp"><em>N</em></code>,
 <code class="sourceCode cpp"><em>A</em></code>,
 <code class="sourceCode cpp"><em>W</em></code>,
-<code class="sourceCode cpp"><em>NUA</em></code>) ([class.mem.general]),
-or <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code>
+<code class="sourceCode cpp"><em>NUA</em></code>), or <code class="sourceCode cpp">dealias<span class="op">(</span>r<span class="op">)</span></code>
 represents a type <code class="sourceCode cpp"><em>T</em></code>, then
 <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">(</span><em>LAT</em><span class="op">)</span></code>
 where <code class="sourceCode cpp"><em>LAT</em></code> is the

2996_reflection/reflection.md

@@ -33,6 +33,7 @@ Since [@P2996R11]:
   * disallow reflection of local parameters introduced by `$requires-expression$`s
 * library wording updates
   * improve specification of `access_context::current()` (including examples)
+  * `size_of(r)` is no longer constant if `r` is a bit-field
 
 Since [@P2996R10]:
 
@@ -7568,9 +7569,9 @@ consteval member_offset offset_of(info r);
 consteval size_t size_of(info r);
 ```
 
-[#]{.pnum} *Constant When*: `dealias(r)` is a reflection of a type, object, value, variable of non-reference type, non-static data member, direct base class relationship, or data member description. If `dealias(r)` represents a type, then `is_complete_type(r)` is `true`.
+[#]{.pnum} *Constant When*: `dealias(r)` is a reflection of a type, object, value, variable of non-reference type, non-static data member that is not a bit-field, direct base class relationship, or data member description (`$T$`, `$N$`, `$A$`, `$W$`, `$NUA$`) ([class.mem.general]) where `$W$` is not ⊥. If `dealias(r)` represents a type, then `is_complete_type(r)` is `true`.
 
-[#]{.pnum} *Returns*: If `r` represents a non-static data member of type `$T$`, a data member description (`$T$`, `$N$`, `$A$`, `$W$`, `$NUA$`) ([class.mem.general]), or `dealias(r)` represents a type `$T$`, then `sizeof($LAT$)` where `$LAT$` is the layout-associated type ([class.member.general]) of a non-static data member of type `$T$`. Otherwise, `size_of(type_of(r))`.
+[#]{.pnum} *Returns*: If `r` represents a non-static data member of type `$T$`, a data member description (`$T$`, `$N$`, `$A$`, `$W$`, `$NUA$`), or `dealias(r)` represents a type `$T$`, then `sizeof($LAT$)` where `$LAT$` is the layout-associated type ([class.member.general]) of a non-static data member of type `$T$`. Otherwise, `size_of(type_of(r))`.
 
 [It is possible that while `sizeof(char) == size_of(^^char)` that `sizeof(char&) != size_of(^^char&)`. If `b` represents a direct base class relationship of an empty base class, then `size_of(b) > 0`.]{.note}