<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/148289>148289</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang-tidy] `readability-redundant-inline-specifier`: with modules, `inline` on in-class member function definitions is not always redundant
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang-tidy
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          localspook
      </td>
    </tr>
</table>

<pre>
    Consider this class:
```cpp
struct s {
    void f() {}
};
```
In a non-module world, `f` is implicitly `inline`, and writing the keyword is redundant.
But C++20 adopted [P1779](wg21.link/p1779), which changes the semantics of this code: when `s` is not attached to the global module, `f` is not `inline`; if you want it to be, you have to say so explicitly. This does not apply to defaulted member functions however:
```cpp
struct s {
    inline s() = default; // Still redundant.
};
```
So I believe this check should not trigger outside the global module and when the function isn't defaulted. (We may even want to add a check like `modernize-modules` that *adds* `inline` to ease the transition to modules.)

Godbolt: https://godbolt.org/z/Gv9YGzq9e
</pre>
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