Build failure with Clang 16 (`-Wincompatible-function-pointer-types`)

[thesamesam]

Clang 16 makes -Wincompatible-function-pointer-types an error by default.

ruby-ldap hits the following failure when building with Clang 16:

[...]
clang -I. -I/usr/include/ruby-2.7.0/x86_64-linux -I/usr/include/ruby-2.7.0/ruby/backward -I/usr/include/ruby-2.7.0 -I. -DUSE_OPENLDAP2 -DUSE_OPENLDAP -DLDAP_DEPRECATED -D_REENTRANT -DHAVE_LDAP_H -DHAVE_LBER_H -DHAVE_OPENSSL_SSL_H -DHAVE_OPENSSL_CRYPTO_H -DHAVE_LDAP_INIT -DHAVE_LDAP_SET_OPTION -DHAVE_LDAP_GET_OPTION -DHAVE_LDAP_START_TLS_S -DHAVE_LDAP_MEMFREE -DHAVE_LDAP_PERROR -DHAVE_LDAP_SORT_ENTRIES -DHAVE_LDAP_SASL_BIND_S -DHAVE_LDAP_RENAME_S -DHAVE_LDAP_COMPARE_S -DHAVE_LDAP_ADD_EXT_S -DHAVE_LDAP_COMPARE_EXT_S -DHAVE_LDAP_DELETE_EXT_S -DHAVE_LDAP_MODIFY_EXT_S -DHAVE_LDAP_SEARCH_EXT_S -DHAVE_LDAP_UNBIND_EXT_S -DHAVE_LDAP_SASL_INTERACTIVE_BIND_S -DRUBY_VERSION_CODE=277    -fPIC -O2 -pipe -march=x86-64 -frecord-gcc-switches -fno-diagnostics-color -fmessage-length=0 -Werror=implicit-function-declaration -Werror=implicit-int -Werror=incompatible-function-pointer-types  -o sslconn.o -c sslconn.c
ldap.c:204:30: error: incompatible function pointer types passing 'VALUE (VALUE, VALUE)' (aka 'unsigned long (unsigned long, unsigned long)') to parameter of type 'rb_block_call_func_t' (aka 'unsigned long (*)(unsigned long, unsigned long, int, const unsigned long *, unsigned long)') [-Werror,-Wincompatible-function-pointer-types]
  rb_iterate (rb_each, hash, rb_ldap_hash2mods_i, tmp);
                             ^~~~~~~~~~~~~~~~~~~
/usr/include/ruby-2.7.0/ruby/ruby.h:1986:60: note: passing argument to parameter here
VALUE rb_iterate(VALUE(*)(VALUE),VALUE,rb_block_call_func_t,VALUE);
                                                           ^

To reproduce this bug:

• with Clang 16, just build normally
• with Clang 15, pass -Werror=incompatible-function-pointer-types
• with GCC, pass -Werror=incompatible-pointer-types (GCC lacks a more specific warning)

Reported downstream in Gentoo at https://bugs.gentoo.org/884405.

[graaff]

The quick fix we applied in Gentoo is to cast rb_ldap_hash2mods_i to (rb_block_call_func_t), but the compiler output also shows:

ldap.c:204:3: warning: ‘rb_iterate’ is deprecated: by: rb_block_call since 1.9 [-Wdeprecated-declarations]

So a more robust fix would probably involve moving to rb_block_call.

[laokz]

With gcc14, it also failed with:

[  898s] ldap.c:204:30: error: passing argument 3 of 'rb_iterate' from incompatible
[  898s] pointer type [-Wincompatible-pointer-types]
[  898s]   204 |   rb_iterate (rb_each, hash, rb_ldap_hash2mods_i, tmp);
[  898s]       |                              ^~~~~~~~~~~~~~~~~~~
[  898s]       |                              |
[  898s] |                              VALUE (*)(VALUE,  VALUE) {aka long unsigned
[  898s] int (*)(long unsigned int,  long unsigned int)}
[  898s] /usr/include/ruby/internal/iterator.h:283:75: note: expected
[  898s] 'rb_block_call_func_t' {aka 'long unsigned int (*)(long unsigned int,  long
[  898s] unsigned int,  int,  const long unsigned int *, long unsigned int)'} but
[  898s] argument is of type 'VALUE (*)(VALUE,  VALUE)' {aka 'long unsigned int (*)(long
[  898s] unsigned int,  long unsigned int)'}
[  898s] 283 | VALUE rb_iterate(VALUE (*func1)(VALUE), VALUE data1,
[  898s] rb_block_call_func_t proc, VALUE data2);
[  898s] |                                                     
[  898s] ~~~~~~~~~~~~~~~~~~~~~^~~~