[clang] [clang-cl] Add support for [[msvc::constexpr]] C++11 attribute (PR #71300)

[Richard Dzenis via cfe-commits]

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@@ -7337,6 +7337,27 @@ static void handleDeclspecThreadAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) ThreadAttr(S.Context, AL));
 }
 
+static void handleMSConstexprAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (!S.getLangOpts().isCompatibleWithMSVC(LangOptions::MSVC2022_3)) {
+    S.Diag(AL.getLoc(), diag::warn_unknown_attribute_ignored)
+        << AL << AL.getRange();
+    return;
+  }
+  auto *FD = cast<FunctionDecl>(D);
+  if (FD->isConstexprSpecified() || FD->isConsteval()) {
+    S.Diag(AL.getLoc(), diag::err_ms_constexpr_not_distinct)
+        << FD->isConsteval() << FD;
+    return;
+  }
+  if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
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RIscRIpt wrote:

Prior to C++20 `constexpr` functions were not allowed to be `virtual` [cppreference](https://en.cppreference.com/w/cpp/language/constexpr).
I know this is an attribute, but this matches MSVC behavior: https://godbolt.org/z/snn6c1EW7

https://github.com/llvm/llvm-project/pull/71300