[clang] [clang] Migrate away from PointerUnion::dyn_cast (NFC) (PR #124425)

[Nikita Popov via cfe-commits]

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@@ -2009,7 +2009,8 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
   /// Retrieve the template argument list as written in the sources,
   /// if any.
   const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
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nikic wrote:

I'm a bit confused by this one. I'd have expected dyn_cast to work here, given the following cast. Or is this the weird case where cast on the non-first pointer union member accepts null?

https://github.com/llvm/llvm-project/pull/124425