[clang] [clang] Fix self-capturing `__block` variables (PR #89475)

[via cfe-commits]

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@@ -2764,6 +2794,9 @@ void CodeGenFunction::emitByrefStructureInit(const AutoVarEmission &emission) {
     auto layoutInfo = CGM.getObjCRuntime().BuildByrefLayout(CGM, type);
     storeHeaderField(layoutInfo, getPointerSize(), "byref.layout");
   }
+
+  if (emission.NeedsInitOnHeap)
+    emitByrefInitOnHeap(pointer);
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ille-apple wrote:

Updated to add the flag.  It's not all that bad.

In fact, if, as I suggested in my last comment, we want to support the byref lingering after unwinding, then there *must* be some kind of runtime state – to track whether whoever drops the last reference needs to run the destructor.  It would be more natural for the Blocks runtime to track that state rather than relying on compiler-generated code, but the state has to live somewhere.

If, on the other hand, we don't want to support the byref lingering after unwinding, then I could go back to the idea of mutating the existing `flags` field.  In that scenario, the existence of runtime state would be unnecessary, but the implementation would be relatively unobtrusive.

https://github.com/llvm/llvm-project/pull/89475