[clang] [clang-tools-extra] [clang-tidy][dataflow] Add `bugprone-null-check-after-dereference` check (PR #84166)

[via cfe-commits]

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+.. title:: clang-tidy - bugprone-null-check-after-dereference
+
+bugprone-null-check-after-dereference
+=====================================
+
+.. note::
+
+   This check uses a flow-sensitive static analysis to produce its
+   results. Therefore, it may be more resource intensive (RAM, CPU) than the
+   average clang-tidy check.
+
+This check identifies redundant pointer null-checks, by finding cases where the
+pointer cannot be null at the location of the null-check.
+
+Redundant null-checks can signal faulty assumptions about the current value of
+a pointer at different points in the program. Either the null-check is
+redundant, or there could be a null-pointer dereference earlier in the program.
+
+.. code-block:: c++
+
+   int f(int *ptr) {
+     *ptr = 20; // note: one of the locations where the pointer's value cannot be null
+     // ...
+     if (ptr) { // bugprone: pointer is checked even though it cannot be null at this point
+       return *ptr;
+     }
+     return 0;
+   }
+
+Supported pointer operations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Pointer null-checks
+-------------------
+
+The checker currently supports null-checks on pointers that use
+``operator bool``, such as when being used as the condition
+for an `if` statement.
----------------
martinboehme wrote:

Is `p != nullptr` supported too? This is a common construct (and is mandated over `operator bool` by some style guides)l

https://github.com/llvm/llvm-project/pull/84166