I did this

Built a Debug version of curl using gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0.

Passed the -D ENABLE_DEBUG=1 flag to CMake to enable assertions in libcurl.

Modified docs/examples/https.c to set CURLOPT_SSL_SESSIONID_CACHE to zero, as follows:

int main(void)
{
  CURL *curl;
  CURLcode res;

  curl_global_init(CURL_GLOBAL_DEFAULT);

  curl = curl_easy_init();
  if(curl) {

    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");

    // Added this line
    curl_easy_setopt(curl, CURLOPT_SSL_SESSIONID_CACHE, 0L);

Built https.c with this command:

gcc https.c -I ../../include/ -L ../../build/Debug/lib -l:libcurl-d.so

When this code ran, I received this error:

a.out: /home/jim/repos/curl/lib/vtls/vtls.c:424: Curl_ssl_getsessionid: Assertion `((CURLPROXY_HTTPS == conn->http_proxy.proxytype && ssl_connection_complete != conn->proxy_ssl[conn->sock[1] == -1 ? 0 : 1].state) ? data->set.proxy_ssl.primary.sessionid : data->set.ssl.primary.sessionid)' failed.
Aborted

Reviewing vtls.c line 424, it is asserting on the value of sessionid, which is correctly set to zero because of the call to curl_easy_setopt(curl, CURLOPT_SSL_SESSIONID_CACHE, 0L);. The zero is interpreted as a boolean false and the assertion fails.

It appears that Line 424 should be removed entirely because it's handled properly on line 426.

I expected the following

I expected the sample to run correctly and print the content from example.com.

curl/libcurl version

Using commit 161cbc502ee0bdbda052d6da17d24fa7835f83e7, but this problem goes back to at least curl-7.79.1 .

operating system

Linux mynode 5.10.16.3-microsoft-standard-WSL2 #1 SMP Fri Apr 2 22:23:49 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux