RIscRIpt wrote:
Initially I replicated semantics of `[[msvc::constexpr]]` from MSVC, so that it
was possible to use it the same way as in MSVC, even `[[msvc::constexpr]]
return ::new` from non-std namespace. E.g. https://godbolt.org/z/7eKh5Envz
```cpp
// RUN: %clang_cc1 -fms-compatibility -fms-compatibility-version=19.33
-std=c++20 -ast-dump %s | FileCheck %s
// CHECK: used operator new
// CHECK: MSConstexprAttr 0x{{[0-9a-f]+}} <col:17, col:23>
[[nodiscard]] [[msvc::constexpr]] inline void* __cdecl operator
new(decltype(sizeof(void*)), void* p) noexcept { return p; }
// CHECK: used constexpr construct_at
// CHECK: AttributedStmt 0x{{[0-9a-f]+}} <col:46, col:88>
// CHECK-NEXT: MSConstexprAttr 0x{{[0-9a-f]+}} <col:48, col:54>
// CHECK-NEXT: ReturnStmt 0x{{[0-9a-f]+}} <col:66, col:88>
constexpr int* construct_at(int* p, int v) { [[msvc::constexpr]] return ::new
(p) int(v); }
constexpr bool check_construct_at() { int x; return *construct_at(&x, 42) ==
42; }
static_assert(check_construct_at());
```
However @rsmith raised a concern over changes in
`PointerExprEvaluator::VisitCXXNewExpr`:
```diff
bool IsNothrow = false;
bool IsPlacement = false;
+ bool IsMSConstexpr = Info.CurrentCall->CanEvalMSConstexpr &&
+ OperatorNew->hasAttr<MSConstexprAttr>();
if (OperatorNew->isReservedGlobalPlacementOperator() &&
- Info.CurrentCall->isStdFunction() && !E->isArray()) {
+ (Info.CurrentCall->isStdFunction() || IsMSConstexpr) && !E->isArray()) {
```
> Do we really need this change? Was our existing check of whether the caller
> is in namespace std not sufficient for MS' standard library? I'd strongly
> prefer not to have a documented, user-visible attribute that gives permission
> to use placement new directly.
If I could choose, I would opt to fully replicate the behavior of MSVC.
However, I also acknowledge the concerns that have been raised.
@AaronBallman, @erichkeane, any comments?
https://github.com/llvm/llvm-project/pull/71300
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