[clang-tools-extra] [clang-tidy] Add `readability-redundant-nested-if` check (PR #181558)

[via cfe-commits]

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+.. title:: clang-tidy - readability-redundant-nested-if
+
+readability-redundant-nested-if
+===============================
+
+Finds nested ``if`` statements that can be merged by combining their conditions
+with ``&&``.
+
+Example:
+
+.. code-block:: c++
+
+  if (a) {
+    if (b) {
+      work();
+    }
+  }
+
+becomes
+
+.. code-block:: c++
+
+  if ((a) && (b)) {
+    work();
+  }
+
+The check can merge longer chains as well:
+
+.. code-block:: c++
+
+  if (a) {
+    if (b) {
+      if (c) {
+        work();
+      }
+    }
+  }
+
+becomes
+
+.. code-block:: c++
+
+  if ((a) && (b) && (c)) {
+    work();
+  }
+
+The check also supports outer declaration conditions in C++17 and later:
+
+.. code-block:: c++
+
+  if (bool x = ready()) {
+    if (can_run()) {
+      work();
+    }
+  }
+
+becomes
+
+.. code-block:: c++
+
+  if (bool x = ready(); x && (can_run())) {
+    work();
+  }
+
+Safety rules
+------------
+
+The check only transforms chains where:
+
+- Neither outer nor nested ``if`` has an ``else`` branch.
+
+- Nested merged ``if`` statements do not use condition variables.
+
+- In C++17 and later, the outermost ``if`` may use a condition variable if it
+  can be rewritten to an init-statement form, for example
+  ``if (auto v = f())`` to ``if (auto v = f(); v && ...)``.
+
+- When the outermost statement is already in ``if (init; cond)`` form, the
+  check keeps ``init`` unchanged and merges only into ``cond``.
+
+- By default, merged conditions avoid user-defined ``bool`` conversions to
+  preserve short-circuit semantics. This can be changed with
+  :option:`UserDefinedBoolConversionMode`.
+
+- Only the outermost ``if`` may have an init-statement.
+
+- No merged ``if`` is ``if consteval``.
+
+- All merged ``if`` statements are either all ``if constexpr`` or all regular
+  ``if``.
+
+- No merged ``if`` statement has statement attributes.
+
+- All rewritten ranges are free of macro/preprocessor-sensitive edits.
+
+- Fix-its are suppressed when comments in removed nested headers cannot be
+  preserved safely. Comments inside conditions are preserved, while
+  other comments between the ``ifs`` disable fix-its.
+
+For ``if constexpr``, nested merged conditions must be
+non-instantiation-dependent to avoid template semantic changes. The outermost
+condition may be instantiation-dependent when all nested merged conditions are
+constant ``true``.
+
+Options
+-------
+
+.. option:: UserDefinedBoolConversionMode
+
+   Controls how chains with an outer condition that relies on user-defined
+   ``bool`` conversion are handled.
+
+   - `None`
+     No diagnostic is emitted for those chains.
+   - `WarnOnly`
+     Emit diagnostics, but do not provide fix-its.
+   - `WarnAndFix`
----------------
EugeneZelenko wrote:

```suggestion

   - `WarnAndFix`
```

https://github.com/llvm/llvm-project/pull/181558