[clang-tidy][docs][NFC] Improve documentation of bugprone-unhandled-e…

…xception-at-new

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D114602

clang-tools-extra/docs/clang-tidy/checks/bugprone-unhandled-exception-at-new.rst

@@ -5,21 +5,51 @@ bugprone-unhandled-exception-at-new
 
 Finds calls to ``new`` with missing exception handler for ``std::bad_alloc``.
 
+Calls to ``new`` may throw exceptions of type ``std::bad_alloc`` that should
+be handled. Alternatively, the nonthrowing form of ``new`` can be
+used. The check verifies that the exception is handled in the function
+that calls ``new``.
+
+If a nonthrowing version is used or the exception is allowed to propagate out
+of the function no warning is generated.
+
+The exception handler is checked if it catches a ``std::bad_alloc`` or
+``std::exception`` exception type, or all exceptions (catch-all).
+The check assumes that any user-defined ``operator new`` is either
+``noexcept`` or may throw an exception of type ``std::bad_alloc`` (or one
+derived from it). Other exception class types are not taken into account.
+
 .. code-block:: c++
 
   int *f() noexcept {
-    int *p = new int[1000];
+    int *p = new int[1000]; // warning: missing exception handler for allocation failure at 'new'
+    // ...
+    return p;
+  }
+
+.. code-block:: c++
+
+  int *f1() { // not 'noexcept'
+    int *p = new int[1000]; // no warning: exception can be handled outside
+                            // of this function
+    // ...
+    return p;
+  }
+
+  int *f2() noexcept {
+    try {
+      int *p = new int[1000]; // no warning: exception is handled
+      // ...
+      return p;
+    } catch (std::bad_alloc &) {
+      // ...
+    }
+    // ...
+  }
+
+  int *f3() noexcept {
+    int *p = new (std::nothrow) int[1000]; // no warning: "nothrow" is used
     // ...
     return p;
   }
 
-Calls to ``new`` can throw exceptions of type ``std::bad_alloc`` that should
-be handled by the code. Alternatively, the nonthrowing form of ``new`` can be
-used. The check verifies that the exception is handled in the function
-that calls ``new``, unless a nonthrowing version is used or the exception
-is allowed to propagate out of the function (exception handler is checked for
-types ``std::bad_alloc``, ``std::exception``, and catch-all handler).
-The check assumes that any user-defined ``operator new`` is either
-``noexcept`` or may throw an exception of type ``std::bad_alloc`` (or derived
-from it). Other exception types or exceptions occurring in the object's
-constructor are not taken into account.