[clang-tools-extra] [clang-tidy] Add misc-shadowed-namespace-function check (PR #168406)

[via cfe-commits]

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+.. title:: clang-tidy - misc-shadowed-namespace-function
+
+misc-shadowed-namespace-function
+================================
+
+Detects free functions in the global namespace that shadow functions declared 
+in other namespaces. This check helps prevent accidental shadowing of namespace
+functions, which can lead to confusion about which function is being called and
+potential linking errors.
+
+Examples
+--------
+
+.. code-block:: c++
+
+  namespace utils {
+    void process();
+    void calculate();
+  }
+
+  // Warning: free function shadows utils::process
+  void process() {} 
+
+  // No warning - static function
+  static void calculate() {}
+
+The check will suggest adding the appropriate namespace qualification:
+
+.. code-block:: diff
+
+  - void process() {}
+  + void utils::process() {}
+
+The check will not warn about:
+
+- Static functions or member functions;
+- Functions in anonymous namespaces;
+- The ``main`` function.
+
+Limitations
+-----------
+
+- Does not warn about friend functions:
+
+.. code-block:: c++
+
+  namespace llvm::gsym {
+    struct MergedFunctionsInfo {
+        friend bool operator==(const MergedFunctionsInfo &LHS,
+                               const MergedFunctionsInfo &RHS);
+    };
+  }
+
+  using namespace llvm::gsym;
+
+  bool operator==(const MergedFunctionsInfo &LHS,  // no warning in this version
+                  const MergedFunctionsInfo &RHS) {
+    return LHS.MergedFunctions == RHS.MergedFunctions;
+  }
+
+- Does not warn about template functions
+- Does not warn about variadic functions
+
----------------
EugeneZelenko wrote:

Excessive newline.

https://github.com/llvm/llvm-project/pull/168406