[clang-tools-extra] [clang-tidy] Allow use of decltype(nullptr) for defining nullptr_t (PR #231637)

[via cfe-commits]

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@@ -18,3 +18,15 @@ template<class T=decltype(nullptr)>
 struct bar {};
 // CHECK-MESSAGES: :[[@LINE-2]]:18: warning: use std::nullptr_t instead
 // CHECK-FIXES: template<class T=std::nullptr_t>
+
+typedef decltype(nullptr) nullptr_t;
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serge-sans-paille wrote:

Question: that's to support C23's `nullptr_t`, right?

https://github.com/llvm/llvm-project/pull/231637