$ g++-trunk -v
Using built-in specs.
COLLECT_GCC=g++-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto --prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160918 (experimental) [trunk revision 240220] (GCC)
$
$ g++-trunk small.C
small.C:2:35: error: an explicit specialization must be preceded by ‘template <>’
 template <class T, int a, class U struct B<T, a, U, 1> {};
                                   ^~~~~~~~~~~~~~~~~~~~
                                   template <>
small.C:2:42: error: enclosing class templates are not explicitly specialized
 template <class T, int a, class U struct B<T, a, U, 1> {};
                                          ^~~~~~~~~~~~~
small.C:2:56: error: types may not be defined in parameter types
 template <class T, int a, class U struct B<T, a, U, 1> {};
                                                        ^
small.C:2:56: error: definition of ‘struct B<T, a, U, 1>’ inside template parameter list
small.C:2:57: error: two or more data types in declaration of ‘parameter’
 template <class T, int a, class U struct B<T, a, U, 1> {};
                                                         ^
small.C:2:58: error: expected ‘>’ before ‘;’ token
 template <class T, int a, class U struct B<T, a, U, 1> {};
                                                          ^
small.C:2:58: error: expected unqualified-id before ‘;’ token
small.C:3:20: internal compiler error: tree check: accessed elt 1 of tree_vec with 0 elts in unify, at cp/pt.c:19674
 B<int, 2, char, 1> i;
                    ^
0x107f2b5 tree_vec_elt_check_failed(int, int, char const*, int, char const*)
        ../../gcc-source-trunk/gcc/tree.c:9939
0x6f2572 tree_vec_elt_check
        ../../gcc-source-trunk/gcc/tree.h:3247
0x6f2572 unify
        ../../gcc-source-trunk/gcc/cp/pt.c:19674
0x6f1915 unify
        ../../gcc-source-trunk/gcc/cp/pt.c:20081
0x6f4156 get_partial_spec_bindings
        ../../gcc-source-trunk/gcc/cp/pt.c:20851
0x6eb961 most_specialized_partial_spec
        ../../gcc-source-trunk/gcc/cp/pt.c:21107
0x707005 instantiate_class_template_1
        ../../gcc-source-trunk/gcc/cp/pt.c:9848
0x707005 instantiate_class_template(tree_node*)
        ../../gcc-source-trunk/gcc/cp/pt.c:10416
0x7adbcb complete_type(tree_node*)
        ../../gcc-source-trunk/gcc/cp/typeck.c:133
0x66b315 start_decl_1(tree_node*, bool)
        ../../gcc-source-trunk/gcc/cp/decl.c:5168
0x6944bf start_decl(cp_declarator const*, cp_decl_specifier_seq*, int, tree_node*, tree_node*, tree_node**)
        ../../gcc-source-trunk/gcc/cp/decl.c:5131
0x79a089 cp_parser_init_declarator
        ../../gcc-source-trunk/gcc/cp/parser.c:18701
0x79aae7 cp_parser_simple_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12495
0x79ae66 cp_parser_block_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12363
0x7a3d14 cp_parser_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12260
0x7a28af cp_parser_declaration_seq_opt
        ../../gcc-source-trunk/gcc/cp/parser.c:12139
0x7a2bf8 cp_parser_translation_unit
        ../../gcc-source-trunk/gcc/cp/parser.c:4356
0x7a2bf8 c_parse_file()
        ../../gcc-source-trunk/gcc/cp/parser.c:37714
0x917b92 c_common_parse_file()
        ../../gcc-source-trunk/gcc/c-family/c-opts.c:1073
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.
$
$ cat small.C
template <class, int, class, int> struct B {};
template <class T, int a, class U struct B<T, a, U, 1> {};
B<int, 2, char, 1> i;
$