<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/125491>125491</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            if constexpr code not ignored
        </td>
    </tr>

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

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

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

<pre>
    ### Minimal reproducible example
```cpp
template <class... Ts>
struct Type_list {
        private:
        template <int index, class... Args>
        struct Get_at;
        template <class T, class... Args>
        struct Get_at<0, T, Args...> {
                using Type = T;
        };
        template <int index, class T, class... Args>
        struct Get_at<index, T, Args...> {
                using Type = typename Get_at<index - 1, Args...>::Type;
        };

        public:
        template <int index>
        using at = typename Get_at<index, Ts...>::Type;
        constexpr static auto size = sizeof...(Ts);
};

template <class F>
struct CI {};
template <class R, class... PArgs>
struct CI<R (*)(PArgs...)> {
        using Args = Type_list<PArgs...>;
};

template <class Function>
constexpr bool is_viable_source = []<class Properties_list>(Properties_list) {
        if constexpr (Properties_list::size != Properties_list::size) {
                static_assert(false);
                return sizeof(typename CI<Function>::Args::template at<0>) > 0;
        }
        return false;
}(Type_list<>{});

int main() {
        return is_viable_source<void (*)()>;
}

```
https://godbolt.org/z/o3co4j31e

### Expected behavior
Successfully builds executable equivalent to `int main() {}`.

### Actual behavior: Compilation error
```
<source>:17:2: error: implicit instantiation of undefined template 'Type_list<>::Get_at<0>'
   17 |         using at = typename Get_at<index, Ts...>::Type;
      | ^
<source>:32:55: note: in instantiation of template type alias 'at' requested here
   32 |                 return sizeof(typename CI<Function>::Args::template at<0>) > 0;
      | ^
<source>:29:78: note: while substituting into a lambda expression here
   29 | constexpr bool is_viable_source = []<class Properties_list>(Properties_list) {
      | ^
<source>:38:9: note: in instantiation of variable template specialization 'is_viable_source<void (*)()>' requested here
   38 |         return is_viable_source<void (*)()>;
      |                ^
<source>:5:9: note: template is declared here
    5 |         struct Get_at;
      | ^
1 error generated.
```

### Affected versions
Clang 18, clang 19 and clang trunk
C++23 and above (it seems the explicit lambda template argument is required to reproduce, which is a C++23 feature)
Gcc compiles the code successfully.

### Reasoning why clang is wrong
The error is caused by code inside an `if constexpr` that evaluates to `false`, thus clang should not have attempted to compile the code or ignored the error. Clang partially agrees that the `if constexpr` evaluates to `false` and that the code should not be compiled since it does not complain about the `static_assert(false);`.


</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJy0V09v2z4S_TT0ZVBDomzLPvigOHGxhwWCbu4BJY0tdmlS5R8n6adfDCVLtuM07f5QI0Dlajjz3rzhIy2ck3uNuGbzOza_n4jgG2PXTwb3aCelqd_WjGfdH_xbankQCiy21tShkqVCwFdxaBWypGCLpPur2pYlhcdDq4RHYNmmUsK56XQKT45lDywpnLeh8vD01uKzks4Dy-8oRbJqrTwKjywruu_naaT2IHWNr4xvYMhZ2H2flSWrPvFX9M_Cs-zufZK4Dp5-O8UmodAYT2HT6ZRlDyNelqyCk3ofuQDL7uFpKMvy-5sQ3vP4EzzDyj_A5N9a1OKAV1ngC6SXSajvWUHrbrGIAoVSyeoTfU7YOxjC_wpEpPJx-cpo5_G1teC88LICEbwBJ392zOjB7Gg1Xz45xlf9wjPQ79XfXkzh5l-xdacV78O_XYjzOKozJGDZ5hswvmS8IAh8-XhqKQE6k6ZrCL3sZuW0AVi2eTxX4Tc4BF15aXSHZOxSaYwC6Z6PUpQKn50Jtupa1e3xIcGjNS1aL9H1CB4I99V_8tWIXe5gLHMjNmrXCcNTKvhhwEXaOOKk7LNwDq1nfLkTyuGoZYyx6IPVvd6ML4dpit0_70Ys04lET0Pr-t1MPFdAqiQXMx4f-iodgEEFvjwXijJ08zIiLGj2D0LqOARn7PqE14KwbHM0sr6YmW5Wxqrdjju5KkuKxvs2kuJbxrd7U5dG-amxe8a3Pxnfmqwys-9Ziv3SwbkfXlusPNZQYiOO0liWFP8JVYXO7YJSb1AGqWoH-IpV8CL6-o8gj0Kh9uANsEVyg2B-zxbJ9LpYUfkg1FgqK2BjDq1UggQCtDbWPyfGss2pLSRfmrOs4LSwC84KkIdWyUqSvzgvtJddMrODoGvcSY01jHuE59d6xUk4s3QagpwlBQCkObB8A6fPP7WsLgtlZPOH99wy4jWfEydt4jkHUr9nNXAhBCCUFI5oCc94DhZ_BHSkZ4MWu5oZv2Bx-vy9XfMJT5rlIl-e83xppEJwoXRe-uCpzVJ7AwKUOJS1ADIWdI5aMBKLs7aBv25wn-lGVFafyXYUNqIa9XMtVlIo-bOLYTz_Ayv4SOvlhdb_p8WMhK8-N_nPr-kPDKWDGisl7AVGmF-kvnEvu2542u122KNGKzzW02uXuHCZ3a6ztCNamhjHkmKjhN5DuuyPa3pegdB1_8XboP9LYYzfMX7Hs_hOlOZIlrGUHhziwYFv6Frb-00_muN2sPtwIFeULmojibY3w6UYqfhLI6uGIgSMxXYofLDxWEuKr1UFVXRF7ApWpqa9MXryO1_9hsIZTbvmpXnrKUkHL9boPUuKJ0IdGygdVCI4svu3Lq_UTtYIQkcjPzvG2SIB3wgPeBQqCE9gott3B-Ai3n19E1xfzzUmqJpmABpxJHOgxviuBT2fkQ5h2WsTO3RCN4VOpVZYLwWdPWJvMTZB-Bh2A-JH6KKAw8KuhSPCEk-QanBSVwjSQ23Qxbf0SgmpaQDCUPlXN5HhrJvU66xeZSsxwXWaZ8v5cplkfNKss6qeYZ2WyWwm6mxRp8liV6eL2UxUs1nCdxO55gmfJzzJ0izJUz7FXCTZakmhuMhTwWYJHoRUU6WOBzraJ9K5gOuUz2erdKJEicrFX2uca3yB-JZxTj_e7JoWfSnD3rFZQu7mxjReeoXriytc7Be1ohdpEqxaX90xpG9COa3MgfEt5er_-dJa8x0rz_g2InCMb3uIxzX_XwAAAP__CY5hdQ">