[clang-tools-extra] [clang-tidy] Add new check bugprone-unintended-char-ostream-output (PR #127720)

[Carlos Galvez via cfe-commits]

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@@ -0,0 +1,45 @@
+.. title:: clang-tidy - bugprone-unintended-char-ostream-output
+
+bugprone-unintended-char-ostream-output
+=======================================
+
+Finds unintended character output from ``unsigned char`` and ``signed char`` to an
+``ostream``.
+
+Normally, when ``unsigned char (uint8_t)`` or ``signed char (int8_t)`` is used, it
+is more likely a number than a character. However, when it is passed directly to
+``std::ostream``'s ``operator<<``, the result is the character output instead
+of the numeric value. This often contradicts the developer's intent to print
+integer values.
+
+.. code-block:: c++
+
+  uint8_t v = 65;
+  std::cout << v; // output 'A' instead of '65'
+
+The check will suggest casting the value to an appropriate type to indicate the
+intent, by default, it will cast to ``unsigned int`` for ``unsigned char`` and
+``int`` for ``signed char``.
+
+.. code-block:: c++
+
+  std::cout << static_cast<unsigned int>(v); // when v is unsigned char
+  std::cout << static_cast<int>(v); // when v is signed char
+
+To avoid lengthy cast statements, add prefix ``+`` to the variable can also
+suppress warnings.
----------------
carlosgalvezp wrote:

Would be good to explain that this triggers an implicit promotion to `int` (even if the variable is `unsigned`).

https://github.com/llvm/llvm-project/pull/127720