[Clang][Regression:21] Rejects valid lambda invocation in constexpr if

[MagentaTreehouse]

The following C++17 code should be valid:

template <class>
auto f{[] (auto...) {
    if constexpr ([] (auto) { return true; }(0))
        return 0;
}};

int main() {
    f<int>();
}

Error since Clang 21:

<source>:3:19: error: no matching function for call to object of type '(lambda at <source>:3:19)'
    3 |     if constexpr ([] (auto) { return true; }(0))
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:2:21: note: while substituting into a lambda expression here
    2 | auto f{[] (auto...) {
      |                     ^
<source>:8:5: note: in instantiation of variable template specialization 'f' requested here
    8 |     f<int>();
      |     ^
<source>:3:19: note: candidate template ignored: couldn't infer template argument 'auto:1'
    3 |     if constexpr ([] (auto) { return true; }(0))
      |    

See https://compiler-explorer.com/z/Kx5qWPev7.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (MagentaTreehouse)

The following C++17 code should be valid: ```c++ template <class> auto f{[] (auto...) { if constexpr ([] (auto) { return true; }(0)) return 0; }};

int main() {
f<int>();
}

Error since Clang 21:
```console
<source>:3:19: error: no matching function for call to object of type '(lambda at <source>:3:19)'
    3 |     if constexpr ([] (auto) { return true; }(0))
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:2:21: note: while substituting into a lambda expression here
    2 | auto f{[] (auto...) {
      |                     ^
<source>:8:5: note: in instantiation of variable template specialization 'f' requested here
    8 |     f<int>();
      |     ^
<source>:3:19: note: candidate template ignored: couldn't infer template argument 'auto:1'
    3 |     if constexpr ([] (auto) { return true; }(0))
      |    

See https://compiler-explorer.com/z/Kx5qWPev7.

[keinflue]

Bisects to af2bb8f @cor3ntin