[clang] [Clang] [CodeGen] Perform derived-to-base conversion on explicit object parameter in lambda (PR #89828)

[John McCall via cfe-commits]

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@@ -4684,6 +4684,29 @@ LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field,
     else
       LambdaLV = MakeAddrLValue(AddrOfExplicitObject,
                                 D->getType().getNonReferenceType());
+
+    // Make sure we have an lvalue to the lambda itself and not a derived class.
+    auto *ThisTy = D->getType().getNonReferenceType()->getAsCXXRecordDecl();
+    auto *LambdaTy = cast<CXXRecordDecl>(Field->getParent());
+    if (ThisTy != LambdaTy) {
+      CXXBasePaths Paths(/*FindAmbiguities=*/false, /*RecordPaths=*/true,
+                         /*DetectVirtual=*/false);
+
+      [[maybe_unused]] bool Derived = ThisTy->isDerivedFrom(LambdaTy, Paths);
+      assert(Derived && "Type not derived from lambda type?");
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rjmccall wrote:

We could do either.  Doing it once in the prologue makes some sense to me; in the (very unlikely) case that it's not just a static offset, that would probably lead to better code.

https://github.com/llvm/llvm-project/pull/89828