[clang] [Clang] Diagnose down casting static_cast (PR #117914)

[Erich Keane via cfe-commits]

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@@ -7982,6 +7982,9 @@ def err_bad_reinterpret_cast_reference : Error<
 def warn_undefined_reinterpret_cast : Warning<
   "reinterpret_cast from %0 to %1 has undefined behavior">,
   InGroup<UndefinedReinterpretCast>, DefaultIgnore;
+def warn_static_downcast : Warning<
+  "static downcast from %0 to %1 is potentially dangerous%select{|; did you mean 'dynamic_cast'?}2">,
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erichkeane wrote:

I think it is more than 'potentially dangerous', yes?  Perhaps also we should mention the polymorphic-ness of the types as to why it is dangerous.

https://github.com/llvm/llvm-project/pull/117914