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

[Kazu Hirata 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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kazutakahirata wrote:

I also thought so, but surprisingly, `cast<T *>(PU)`, where `PU` satisfies `PU.isNull()`, seems to work.  I don't know if being "non-first" matters here.

For this reason, I'm migrating `PU.dyn_cast<T *>()` to `dyn_cast<T *>(PU)` if the if-then-else chain ends with some sort of dereference like `*cast<U *>(PU)` or `cast<U *>(PU)->foo()`.


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