[clang] Allow constexpr cast from "void*" in more cases

When converting from "cv void*" to "T*" in a constant expression, check
that the actual pointee type is similar to "T" (as opposed to requiring
that the types are the same except top-level qualifiers).

Also implement the resolution of the tentatively ready CWG issue 2819,
adding support for the case when the operand is a null pointer.

Author: offsetof

clang/lib/AST/ExprConstant.cpp

@@ -9237,9 +9237,10 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
       bool HasValidResult = !Result.InvalidBase && !Result.Designator.Invalid &&
                             !Result.IsNullPtr;
       bool VoidPtrCastMaybeOK =
-          HasValidResult &&
-          Info.Ctx.hasSameUnqualifiedType(Result.Designator.getType(Info.Ctx),
-                                          E->getType()->getPointeeType());
+          Result.IsNullPtr ||
+          (HasValidResult &&
+           Info.Ctx.hasSimilarType(Result.Designator.getType(Info.Ctx),
+                                   E->getType()->getPointeeType()));
       // 1. We'll allow it in std::allocator::allocate, and anything which that
       //    calls.
       // 2. HACK 2022-03-28: Work around an issue with libstdc++'s

clang/test/CXX/drs/dr28xx.cpp

@@ -10,6 +10,14 @@
 // expected-no-diagnostics
 #endif
 
+namespace cwg2819 { // cwg2819: 19 review 2023-12-01
+#if __cpp_constexpr >= 202306L
+  constexpr void* p = nullptr;
+  constexpr int* q = static_cast<int*>(p);
+  static_assert(q == nullptr);
+#endif
+}
+
 namespace cwg2847 { // cwg2847: 19
 
 #if __cplusplus >= 202002L

clang/test/CXX/expr/expr.const/p5-26.cpp

@@ -37,3 +37,10 @@ void err() {
                                                 // cxx23-note {{cast from 'void *' is not allowed in a constant expression in C++ standards before C++2c}} \
                                                 // cxx26-note {{cast from 'void *' is not allowed in a constant expression because the pointed object type 'T' is not similar to the target type 'S'}}
 }
+
+int* p;
+constexpr int** pp = &p;
+constexpr void* vp = pp;
+constexpr auto cvp = static_cast<const int* volatile*>(vp);
+// cxx23-error@-1 {{constant expression}}
+// cxx23-note@-2 {{cast from 'void *' is not allowed in a constant expression}}

clang/www/cxx_dr_status.html

@@ -16722,7 +16722,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2819.html">2819</a></td>
     <td>review</td>
     <td>Cast from null pointer value in a constant expression</td>
-    <td align="center">Not resolved</td>
+    <td title="Clang 19 implements 2023-12-01 resolution" align="center">Not Resolved*</td>
   </tr>
   <tr class="open" id="2820">
     <td><a href="https://cplusplus.github.io/CWG/issues/2820.html">2820</a></td>