[clang] [Clang] Unify interface for accessing template arguments as written for class/variable template specializations (PR #81642)

[Krystian Stasiowski via cfe-commits]

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@@ -1792,23 +1807,11 @@ class ClassTemplateSpecializationDecl
   llvm::PointerUnion<ClassTemplateDecl *, SpecializedPartialSpecialization *>
     SpecializedTemplate;
 
-  /// Further info for explicit template specialization/instantiation.
-  struct ExplicitSpecializationInfo {
-    /// The type-as-written.
-    TypeSourceInfo *TypeAsWritten = nullptr;
-
-    /// The location of the extern keyword.
-    SourceLocation ExternLoc;
-
-    /// The location of the template keyword.
-    SourceLocation TemplateKeywordLoc;
-
-    ExplicitSpecializationInfo() = default;
-  };
-
   /// Further info for explicit template specialization/instantiation.
   /// Does not apply to implicit specializations.
-  ExplicitSpecializationInfo *ExplicitInfo = nullptr;
+  llvm::PointerUnion<const ASTTemplateArgumentListInfo *,
+                     ExplicitInstantiationInfo *>
+      ExplicitInfo = nullptr;
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sdkrystian wrote:

> I spent some time to understand `const ASTTemplateArgumentListInfo *` is only meaningful for partial specializations. 

Not quite... `const ASTTemplateArgumentListInfo *` is also used for explicit specializations. `ExplicitInstantiationInfo *` (should) only be used for explicit instantiations, I just need to remove any instances of `TemplateKeywordLoc` being set for non-explicit-instantiations specializations (planning to do this prior to merging). 

https://github.com/llvm/llvm-project/pull/81642