[clang-tools-extra] [clang-tidy][docs] improve documentation on cppcoreguidelines-narrowing-conversions (#111510) (PR #118209)

[Julian Schmidt via cfe-commits]

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@@ -27,6 +27,39 @@ This check will flag:
  - All applications of binary operators with a narrowing conversions.
    For example: ``int i; i+= 0.1;``.
 
+Arithmetic with smaller integer types than ``int`` trigger implicit conversions,
+as explained under `"Integral Promotion" on cppreference.com
+<https://en.cppreference.com/w/cpp/language/implicit_conversion>`_.
+This check diagnoses more instances of narrowing than the compiler warning
+`-Wconversion` does. The example below demonstrates this behavior.
+
+.. code-block:: c++
+
+   // The following function definition demonstrates usage of arithmetic with
+   // integer types smaller than `int` and how the narrowing conversion happens
+   // implicitly.
+   void computation(short argument1, short argument2) {
+     // Arithmetic written by humans:
+     short result = argument1 + argument2;
+     // Arithmetic actually performed by C++:
+     short result = static_cast<short>(static_cast<int>(argument1) + static_cast<int>(argument2));
+   }
+
+   void recommended_resolution(short argument1, short argument2) {
+     short result = argument1 + argument2;
+     //           ^ warning: narrowing conversion from 'int' to signed type 'short' is implementation-defined
+
+     // The recommended way to resolve this issue with the GSL is one of two ways.
+     // Either by a cast that throws if a loss of precision would occur.
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5chmidti wrote:

IMO the recommendation for the GSL should be done by saying that the `cppcoreguidelines` recommend doing that

https://github.com/llvm/llvm-project/pull/118209