<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/54678>54678</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Diagnostic for constraint satisfication failure can provide irrelevant information
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    Here's an example:

```cpp
template<class>
concept True = true;

template<class>
concept False = false;

template<class>
concept Irrelevant = false;

template<class T> void foo(T t) requires (True<T> || Irrelevant<T>) && False<T> { }
template<class T> void bar(T t) requires (Irrelevant<T> || True<T>) && False<T> { }

int main() {
    foo(42);
    bar(42L);
}
```

The calls to `foo(42)` and `bar(42L)` should (and do) fail, because neither `int` nor `long` satisfy `False`. 

clang currently emits this error for `foo(42)`

```
<source>:14:5: error: no matching function for call to 'foo'
    foo(42);
    ^~~
<source>:10:24: note: candidate template ignored: constraints not satisfied [with T = int]
template<class T> void foo(T t) requires (True<T> || Irrelevant<T>) && False<T> { }
                       ^
<source>:10:72: note: because 'int' does not satisfy 'False'
template<class T> void foo(T t) requires (True<T> || Irrelevant<T>) && False<T> { }
                                                                       ^
<source>:5:17: note: because 'false' evaluated to false
concept False = false;
                ^
```

Which is good.

But it emits this error for `bar(42L)`:

```
<source>:15:5: error: no matching function for call to 'bar'
    bar(42L);
    ^~~
<source>:11:24: note: candidate template ignored: constraints not satisfied [with T = long]
template<class T> void bar(T t) requires (Irrelevant<T> || True<T>) && False<T> { }
                       ^
<source>:11:43: note: because 'long' does not satisfy 'Irrelevant'
template<class T> void bar(T t) requires (Irrelevant<T> || True<T>) && False<T> { }
                                          ^
<source>:8:22: note: because 'false' evaluated to false
concept Irrelevant = false;
                     ^
<source>:11:72: note: and 'long' does not satisfy 'False'
template<class T> void bar(T t) requires (Irrelevant<T> || True<T>) && False<T> { }
                                                                       ^
<source>:5:17: note: because 'false' evaluated to false
concept False = false;
                ^
```

This notes both that `long` does not satisfy `False` but also that `long` does not satisfy `Irrelevant`. But `Irrelevant` is irrelevant - that part of the disjunction holds (because `long` satisfies `True`), so that is not the reason this fails. It's just an irrelevant diagnostic to the user -- which clang (correctly) doesn't provide in the `foo` case.
</pre>
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