<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/99491>99491</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            clang allows incompatible template constructor for class template argument with default value
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          Rush10233
      </td>
    </tr>
</table>

<pre>
    The following code is accepted by clang but rejected by GCC, EDG and MSVC:

```c++
#include<iostream>
template <class K,int NON=0>
struct C    
{      
    int x=0;  
    C<K>(){x=1;}; 
};        
int main(){
    C<int> Cont;
    //Cont.x=1
 std::cout<<Cont.x<<std::endl;
    C<int ,100> Cont_2;
 //Cont_2.x=1
    std::cout<<Cont_2.x<<std::endl;    
}
```
The constructor `C<K>` seems invalid because it does not explain how the class should be instantiated if the default value of `NON` is not taken. For example, in the instantiation of `Cont_2`. As `Cont_2.x` is set to 1, it seems clang will apply the ctor anyway regardless of whether the default argument value is used.

GCC gives a quite puzzling diagnostic about that:

```c++
<source>:6:10: error: expected unqualified-id before ')' token
    6 |     C<K>(){x=1;};
      |          ^
Compiler returned: 1
```

And the diagnostic provided by EDG seems more reasonable:

```c++
"<source>", line 6: error: type used as constructor name does not match type "C<K, NON>"
      C<K>(){x=1;}; 
      ^

1 error detected in the compilation of "<source>".
Compiler returned: 2
```

[https://godbolt.org/z/Gz8cvjaYa](https://godbolt.org/z/Gz8cvjaYa)
</pre>
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