[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Erich Keane via cfe-commits]

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@@ -3141,6 +3148,10 @@ def warn_attribute_ignored_no_calls_in_stmt: Warning<
   "statement">,
   InGroup<IgnoredAttributes>;
 
+def warn_attribute_needs_aggregate : Warning<
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erichkeane wrote:

I still disagree, in that misuse of this attribute should be harsher than a warning, we have precedent for it.  As far as the 'older compiler' thing, typically that is the point of version checks/etc.

That said, I'm LESS Sure than I was before, so I wouldn't mind seeing Aaron's opinion here.

https://github.com/llvm/llvm-project/pull/102040