[clang] [clang] Allow C-style casts in constexpr in MS compatible mode (PR #197005)

[via cfe-commits]

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@@ -15603,3 +15581,17 @@ void ASTContext::recordOffsetOfEvaluation(const OffsetOfExpr *E) {
   if (FieldDecl *FD = Comp.getField(); isPFPField(FD))
     PFPFieldsWithEvaluatedOffset.insert(FD);
 }
+
+bool ASTContext::maybeFoldMSConstexpr(
+    APValue &Val, SmallVectorImpl<PartialDiagnosticAt> &Notes) {
+  if (Notes.size() != 1 || !getLangOpts().MSVCCompat || Val.isLValue())
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eleviant wrote:

Under what circumstances can this happen. To my understanding if evaluator sees lvalue in sub expression then the result is either lvalue or None. The only exception, I've found is label subtraction, e.g `(intptr_t)&&a - (intptr_t)&&b`. I've added few examples demonstrating this behavior in latest commit.

https://github.com/llvm/llvm-project/pull/197005