[clang-tools-extra] [clang-tidy] Add detection of decltype(nullptr) => std::nullptr_t to … (PR #224591)

[Baranov Victor via cfe-commits]

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@@ -292,3 +292,17 @@ template<typename T>
 T *f2(T *a = NULL) {
   return a ? a : NULL;
 }
+
+void foo(decltype(nullptr));
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vbvictor wrote:

Can we write another test-file where will check inclusion of `<cstddef>`

https://github.com/llvm/llvm-project/pull/224591