<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/99572>99572</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Unhelpful diagnostic when decomposing type that doesn't implement all tuple-like operations
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          Michael137
      </td>
    </tr>
</table>

<pre>
    Take this contrived example:
```
struct B { 
  int w;                                
  int z; 
};                                      
 
namespace std { 
template<typename T> struct tuple_size {
}; 
template<>                              
struct tuple_size<B> {                  
    static constexpr int value = 2; 
};                                      
} 
                                        
int main() { 
    auto [x, y] = B{1, 2};              
 return 0;                           
} 
```
We get:
```
decomp.cpp:16:11: error: use of undeclared identifier 'get'
   16 |     auto [x, y] = B{1, 2};
      | ^
decomp.cpp:16:11: note: in implicit initialization of binding declaration 'x'
1 error generated.
```

Compare this to the more helpful error message we get when we do actually implement the `get` operation:
```
struct B { 
  int w;                                                    
  int z;                                                    
  template<int> int get();                                  
  template<> int get<0>() { return w; }                     
  template<> int get<1>() { return z; } 
}; 

$ g++ decomp.cpp -std=c++2a
decomp.cpp:19:11: error: cannot decompose this type; 'std::tuple_element<0UL, B>::type' does not name a type
   19 |     auto [x, y] = B{1, 2};
      | ^
decomp.cpp:19:11: note: in implicit initialization of binding declaration 'x'
1 error generated.
```

I found the second diagnostic to be better at nudging me in the write direction as to what's missing.

Not sure how actionable this is. I just ran into this when trying to write a test for a tuple-like decomposition of a custom type without including the `tuple` header.
</pre>
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