<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/120388>120388</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Suggest include file for popular standard functions
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            enhancement,
            clang:frontend,
            clang:diagnostics
      </td>
    </tr>

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

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

<pre>
    If, e.g. write:
```c++
int main() {
  std::cout << "Hello world\n";
}
```
I think it would be nice for the compiler to suggest importing `<iostream>`. If I later write
```c++
#include <iostream>
int main() {
  std::cout << "Hello world\n";

  int a = 0, b = 0;
  std::memcpy(&a, &b, 4);
}
```
it would be nice if it suggested importing `<cstring>`. I think a simple internal list of popular functions would be enough, we don't need to get fancy here.

I don't know if this collides with functionality that clangd or similar tools already provide though.

Popular D compilers have similar functionality:
```console
~ » cat test.d
void main() {
  writeln("hello world");
}

~ » ldc2 test.d
test.d(7): Error: `writeln` is not defined, perhaps `import std.stdio;` is needed?
```
</pre>
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