hvdijk wrote:
The problem with `union { char x[]; } x = {0};` is in
`ConstStructBuilder::Build` ( `clang/lib/CodeGen/CGExprConstant.cpp`). It does:
```diff
// If this is a union, skip all the fields that aren't being initialized.
if (RD->isUnion() &&
!declaresSameEntity(ILE->getInitializedFieldInUnion(), Field))
continue;
```
But `ILE->getInitializedFieldInUnion()` returns `nullptr` so this skips all
fields. `nullptr` is returned because `InitListChecker::CheckStructUnionTypes`
never calls `StructuredList->setInitializedFieldInUnion`. If that gets called,
then things work.
I think things can be simplified a bit further: if we do
```diff
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 79cf2eed46fe..3fcdf9118468 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2332,7 +2332,7 @@ void InitListChecker::CheckStructUnionTypes(
// We've already initialized a member of a union. We're done.
if (InitializedSomething && RD->isUnion())
- break;
+ return;
// If we've hit the flexible array member at the end, we're done.
if (Field->getType()->isIncompleteArrayType())
```
Then the tests still pass and we can remove or simplify some of the later
checks for unions.
https://github.com/llvm/llvm-project/pull/84428
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