<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/84961>84961</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Code using a lambda to capture a reference to a function rejected with `could not match (null template argument) against (null template argument)`
        </td>
    </tr>

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

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

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

<pre>
    The following code is accepted by other compilers but rejected by clang.  The error message is a bit weird.

```C++
template<typename T>
void g(const T& t) {
}

template<typename T>
void f(const T& t) {
  [t] { g(t); };
}

void h() {
}

int main() {
 f(h);
}
```

Output:
```
<source>:7:9: error: no matching function for call to 'g'
    7 |   [t] { g(t); };
      | ^
<source>:7:7: note: while substituting into a lambda expression here
 7 |   [t] { g(t); };
      |       ^
<source>:14:3: note: in instantiation of function template specialization 'f<void ()>' requested here
   14 |   f(h);
      |   ^
<source>:2:6: note: candidate template ignored: could not match (null template argument) against (null template argument)
    2 | void g(const T& t) {
      | ^
```

Changing the call `g(t)` to be either `g<T>(t)` or `g<void()>(t)` changes the error message to:

```
note: candidate function template not viable: 1st argument ('void (const()') would lose const qualifier
```

Note that the type mentioned here (`void (const()`) has unbalanced parentheses.

Workaround: change the lambda capture from `[t]` to `[t=t]`.

This code was accepted by clang 3.0.0 but is rejected by all version from 3.1 to the latest trunk currently available on Godbolt.

Godbolt link: https://godbolt.org/z/d87rPjja4

</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVk1v4zYQ_TX0ZbCGREmWdfDBcda9tT0E6JmixhKzFOnlR9zsry-Gkr-ySbooGgiJQg5nHt97Q0p4r3qDuGHVA6seFyKGwbqNaFt8fV60tnvdPA0IB6u1PSnTg7QdgvIgpMRjwA7aV7BhQAfSjkel0XloYwCHzyjneamF6ZcAlAmdsw5G9F70UyJoVYATKtctWfbIsu38e5VNz47xB3rSaMDxqEVAVuzC6xGNGBGeWPF1mn2xqoOe8bW0xgd4YnwFgfEGWD2vZ_XjbZFfSHf4LB0Aqx4Cqx5pKFWmeVY8ABUq3i-a0g6Mrz9DpkyAUSjzNizhGaYibxeeGbvN80cMxxhY8ZbV-d9i5210EmnPxbZmxbZhxXZSiV6MhVEEOZD0h2hkUNbAwTqQQmsIFhive8brMx0ANbB6B79GDKQfimfV1w8R1RMQUmkLp0FpBB9bH1SIgXApEywI0GJsOwH499Gh94RzQIdzof-Gan77AFtesmJb3IJTBpTxQZigRGLKHq6snb0G_ohSCa1-TDGM1wdW7JIrJrUpPa_B4feInnroZiMAeTlj-8kJt8A_As1ZsV3dYpbCdKojXBeAqjfWYZdmbdQdxU42IIAmkvLnWOH6OKJJbSF6Qdv_NOiKlCek_96y75nkPafvBmF68kMYcLInW2UXjVcZubVFQJVOqzRX7FKzX0PsZYJw3ahxiZBUBX0qcn-WBXvtsvdQ_kz4z9Ygol-UaHWKzH24UDdZoz67JLE14-M18XVKSmnrESYqv0eh1UGh-4Sz3y3pPoiQ9kNHIFAxZc3sulR2lb1XdpVR2UF4iKYVWhiJHRyFQxMG9OjvTvO_rPsmnI1mclUiMdWcu1aKY4gO4eDsSBLMfTrLdh4oHufBu9xPg_LTvXQS9xdTunigWGbLLF1Kyt_dS-SRF3TprEiFi2VO5SZcAX2A4KL5BjI62pZ-BfEilCZ9wBr4zXat1eEOzDwGWplvtNUhhKMnY_A94_t-XmFdz_j-B-P7bl27P5-fRTktX3SbomuKRixwk9d5xldVWTaLYXNoedPKQ1lh0zZl1eZlU1Vr2cpc4jrDbKE2PONlVuQ8r8sya5YFb9ciw3WbV50oq4aVGY5C6aXWLyMhWCjvI27WZbPKF1q0qH36DODc4AnSJOOcvgrchtZ8aWPvWZlp5YO_ZgkqaNzsiP_oqf8uR3GwF10FODygQyOpUUBcvX8R5KTCQEr__0fOKltEpzdvpFBhiO1S2pHxPW1m_vPl6CwhYnyfKPCM7xNF_wQAAP__clq2ug">