<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/146063>146063</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Regression with lambda return type deduction
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c++17,
            c++20,
            regression,
            rejects-valid,
            lambda,
            constexpr
      </td>
    </tr>

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

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

<pre>
    The following used to compile without diagnostic in Clang 20.1:
```
template<class T, class U>
struct is_same {
  static constexpr bool value = false;
};
 
template<class T>
struct is_same<T, T> {
 static constexpr bool value = true;
};

template <typename T, int N>
struct A {
      static_assert([]() constexpr { // error: value of type 'void' is not contextually convertible to 'bool'
        if constexpr (is_same<T, int>::value)
          return true;
        else
 return (10 != N);
      }());
}; 
int f1() {
    if constexpr (false) {
        A<int, 256> a;
    }
    return 0;
} 
```
but now gives an error on trunk: https://godbolt.org/z/fd5WY4xhY

Note, if you remove the `if constexpr (false)` from `f1()`, then the code compiles. If you add `-> bool` to the lambda, the code also compiles.

The code compiles without diagnostics in GCC, MSVC, and EDG.
</pre>
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