[clang] [clang] Implement `__is_virtual_base_of()` intrinsic (PR #100393)

[via cfe-commits]

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@@ -6027,6 +6027,33 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceI
     return cast<CXXRecordDecl>(rhsRecord->getDecl())
       ->isDerivedFrom(cast<CXXRecordDecl>(lhsRecord->getDecl()));
   }
+  case BTT_IsVirtualBaseOf: {
+    const RecordType *BaseRecord = LhsT->getAs<RecordType>();
+    const RecordType *DerivedRecord = RhsT->getAs<RecordType>();
+
+    if (!BaseRecord || !DerivedRecord) {
+      DiagnoseVLAInCXXTypeTrait(Self, Lhs, tok::kw___is_virtual_base_of);
+      DiagnoseVLAInCXXTypeTrait(Self, Rhs, tok::kw___is_virtual_base_of);
+      return false;
+    }
+
+    if (BaseRecord->isUnionType() || DerivedRecord->isUnionType())
+      return false;
+
+    if (!BaseRecord->isStructureOrClassType() ||
+        !DerivedRecord->isStructureOrClassType())
+      return false;
+
+    if (Self.RequireCompleteType(Rhs->getTypeLoc().getBeginLoc(), RhsT,
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cor3ntin wrote:

> What does it mean for there to be a complete derived class type but an incomplete base class type? Shouldn't this require a complete type in both cases to give reasonable diagnostic behavior?

There is no incomplete base type. We are asking whether some type is a base, and if it isn't, we don't care that it is complete. Type traits don't mandate completeness if they don't need to.

https://github.com/llvm/llvm-project/pull/100393