What is this syntax of wrapping an enum type in parentheses?

There’s this really weird syntax that I don’t understand. On the libcurl site documentation there’s an enum defined as:

typedef enum {
  CURLSSLBACKEND_NONE = 0,
  CURLSSLBACKEND_OPENSSL = 1, /* or one of its forks */
  CURLSSLBACKEND_GNUTLS = 2,
  CURLSSLBACKEND_NSS = 3,
  CURLSSLBACKEND_GSKIT = 5, /* deprecated */
  CURLSSLBACKEND_POLARSSL = 6, /* deprecated */
  CURLSSLBACKEND_WOLFSSL = 7,
  CURLSSLBACKEND_SCHANNEL = 8,
  CURLSSLBACKEND_SECURETRANSPORT = 9,
  CURLSSLBACKEND_AXTLS = 10, /* deprecated */
  CURLSSLBACKEND_MBEDTLS = 11,
  CURLSSLBACKEND_MESALINK = 12, /* deprecated */
  CURLSSLBACKEND_BEARSSL = 13,
  CURLSSLBACKEND_RUSTLS = 14
} curl_sslbackend;

Then it calls a function called "curl_global_sslset() with the enum type wrapped in parentheses minus 1:

 /* list the available ones */
  const curl_ssl_backend **list;
  curl_global_sslset((curl_sslbackend)-1, NULL, &list);

This is really weird syntax to me, and I was surprised to find that:

int r = (curl_sslbackend) - 1; 

Equals -1. But why? curl_sslbackend is a type, how can you use a minus operator on a type? What is happening here?

>Solution :

When you put a type name in parentheses, it’s a type cast. It means to convert the value after it to the specified type.

-1 is not subtraction, it’s a negative integer.

So

int r = (curl_sslbackend) -1; 

means to convert the integer -1 to the curl_sslbackend enum type, then assign this value to the integer r. So you’re converting an integer to an enum, then converting the enum back to an integer. Which is basically a no-op, so it’s the same as

int r = -1;