[clang-tools-extra] [clang-tidy] Unsafe CRTP check (PR #82403)

[via cfe-commits]

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+.. title:: clang-tidy - bugprone-unsafe-crtp
+
+bugprone-unsafe-crtp
+====================
+
+Finds CRTP used in an error-prone way.
+
+If the constructor of a class intended to be used in a CRTP is public, then
+it allows users to construct that class on its own.
+
+Example:
+
+.. code-block:: c++
+
+  template <typename T> class CRTP {
+  public:
+    CRTP() = default;
+  };
+
+  class Good : CRTP<Good> {};
+  Good GoodInstance;
+
+  CRTP<int> BadInstance;
+
+If the constructor is protected, the possibility of an accidental instantiation
+is prevented, however it can fade an error, when a different class is used as
+the template parameter instead of the derived one.
+
+Example:
+
+.. code-block:: c++
+
+  template <typename T> class CRTP {
+  protected:
+    CRTP() = default;
+  };
+
+  class Good : CRTP<Good> {};
+  Good GoodInstance;
+
+  class Bad : CRTP<Good> {};
+  Bad BadInstance;
+
+To ensure that no accidental instantiation happens, the best practice is to make
+the constructor private and declare the derived class as friend.
+
+Example:
+
+.. code-block:: c++
+
+  template <typename T> class CRTP {
+    CRTP() = default;
+    friend T;
+  };
+
+  class Good : CRTP<Good> {};
+  Good GoodInstance;
+
+  class Bad : CRTP<Good> {};
+  Bad CompileTimeError;
+
+  CRTP<int> AlsoCompileTimeError;
----------------
isuckatcs wrote:

I think protected is still as bugprone as public is. It will fade an accidentaly wrong inheritance, e.g.: `class Derived : CRTP<Foo> {};`, I'm not sure if we want to suggest users a fix like this, because it still results in the problem, which we want to fix.

https://github.com/llvm/llvm-project/pull/82403