[Clang] allow usage of placement new operator in [[msvc::constexpr]] context outside of the std namespace

[a-tarasyuk]

Fixes #74924

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #74924

---

Full diff: https://github.com/llvm/llvm-project/pull/119153.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/AST/ExprConstant.cpp (+3-1)
• (added) clang/test/AST/ms-constexpr-new.cpp (+13)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3f58e64cf0ccbc..71666e26146e92 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -525,6 +525,8 @@ Attribute Changes in Clang
 
 - The ``target_version`` attribute is now only supported for AArch64 and RISC-V architectures.
 
+- Clang now permits ``[[msvc::constexpr]]`` usage outside of the std namespace. (#GH74924)
+
 Improvements to Clang's diagnostics
 -----------------------------------
 
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6b5b95aee35522..9dbb350be59091 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -10172,7 +10172,9 @@ bool PointerExprEvaluator::VisitCXXNewExpr(const CXXNewExpr *E) {
       return false;
     IsNothrow = true;
   } else if (OperatorNew->isReservedGlobalPlacementOperator()) {
-    if (Info.CurrentCall->isStdFunction() || Info.getLangOpts().CPlusPlus26) {
+    if (Info.CurrentCall->isStdFunction() || Info.getLangOpts().CPlusPlus26 ||
+        (Info.CurrentCall->CanEvalMSConstexpr &&
+         OperatorNew->hasAttr<MSConstexprAttr>())) {
       if (!EvaluatePointer(E->getPlacementArg(0), Result, Info))
         return false;
       if (Result.Designator.Invalid)
diff --git a/clang/test/AST/ms-constexpr-new.cpp b/clang/test/AST/ms-constexpr-new.cpp
new file mode 100644
index 00000000000000..4b534cf0207644
--- /dev/null
+++ b/clang/test/AST/ms-constexpr-new.cpp
@@ -0,0 +1,13 @@
+// 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());

[cor3ntin]

LGTM

[RIscRIpt]

allow [[msvc::constexpr]] usage outside the std namespace

By the way, to be precisely correct, these changes actually allow usage of "placement new" in [[msvc::constexpr]] context outside of the std namespace.

[RIscRIpt]

LGTM