[clang] Allow constexpr cast from void* in more cases
#89484
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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.
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@llvm/pr-subscribers-clang Author: None (offsetof) Changes[[expr.const]/5.14](https://eel.is/c++draft/expr.const#5.14) says that constexpr cast from <code>cv void*</code> to This patch also allows casting Full diff: https://github.com/llvm/llvm-project/pull/89484.diff 4 Files Affected:
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 73ae8d8efb23a2..d7aee8c92b83d5 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/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
diff --git a/clang/test/CXX/drs/dr28xx.cpp b/clang/test/CXX/drs/dr28xx.cpp
index 4d9b0c76758d53..591bf7abfd872d 100644
--- a/clang/test/CXX/drs/dr28xx.cpp
+++ b/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
diff --git a/clang/test/CXX/expr/expr.const/p5-26.cpp b/clang/test/CXX/expr/expr.const/p5-26.cpp
index 3624b1e5a3e3df..7513b11c09aa01 100644
--- a/clang/test/CXX/expr/expr.const/p5-26.cpp
+++ b/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}}
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 83b71e7c122d1e..2677364065b2f2 100755
--- a/clang/www/cxx_dr_status.html
+++ b/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>
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@offsetof Will you need me to merge that for you? |
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Yes, that would be great, thank you. |
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@offsetof Congratulations on having your first Pull Request (PR) merged into the LLVM Project! Your changes will be combined with recent changes from other authors, then tested Please check whether problems have been caused by your change specifically, as How to do this, and the rest of the post-merge process, is covered in detail here. If your change does cause a problem, it may be reverted, or you can revert it yourself. If you don't get any reports, no action is required from you. Your changes are working as expected, well done! |
[expr.const]/5.14 says that constexpr cast from
cv void*toT*is OK if the pointee type is similar toT, but Clang currently only permits the conversion if the types are the same except top-level cv-qualifiers.This patch also allows casting
(void*)nullptr, implementing the resolution of CWG2819.