<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/137282>137282</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang enhancement] improve -Wreturn-type-c-linkage warning message
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          illusory0x0
      </td>
    </tr>
</table>

<pre>
    When I write the code below, clang report warning, but `struct Foo` is complete type. the Foo<int> doesn't instantiate template. We can instantiate this type in C++ linkage scope, then Most of complier can complies successfully.


### Expected Behavior

add more usefull infomation help user to solve.


```
try to instantiate template in C++ linkage, if this excectly not incomplete type.
```

### Actual Behavior

```
<source>:14:14: warning: 'foo' has C-linkage specified, but returns incomplete type 'Foo<int>' which could be incompatible with C [-Wreturn-type-c-linkage]
```




```C++
template<typename T>
struct Foo {
    T* ptr;
};


void force_instantiate_template() {
    // Foo<int> x;
    // If uncooment the line above, clang woundn't report warning.
    // and MSVC will complie successfully.
}

extern "C" {
 Foo<int> foo() {
        return { .ptr = nullptr } ;
    }

    void bar(Foo<int>) {

 }
}
```



[godbolt](https://godbolt.org/z/e54Gb61Kx)
</pre>
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