The following ill-formed code triggers an ICE in gcc r236239.

$ cat t.cpp
template<typename T>
concept bool C() { return true; }
template bool C<int>(); // expected error: attempt to explicitly instantiate
                        // a function concept.

$ svn info
Path: .
Working Copy Root Path: /home/tom/src/gcc-trunk
URL: svn://gcc.gnu.org/svn/gcc/trunk
Relative URL: ^/trunk
Repository Root: svn://gcc.gnu.org/svn/gcc
Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4
Revision: 236239
Node Kind: directory
Schedule: normal
Last Changed Author: uros
Last Changed Rev: 236238
Last Changed Date: 2016-05-14 05:07:13 -0400 (Sat, 14 May 2016)

$ g++ --version
g++ (GCC) 7.0.0 20160514 (experimental)
...

$ g++ -c -std=c++1z -fconcepts t.cpp
t.cpp:3:22: internal compiler error: in instantiate_decl, at cp/pt.c:21666
 template bool C<int>();
                      ^
0x6a8c51 instantiate_decl(tree_node*, int, bool)
        ../../gcc-trunk/gcc/cp/pt.c:21666
0x77751d cp_parser_explicit_instantiation
        ../../gcc-trunk/gcc/cp/parser.c:15648
0x788c79 cp_parser_declaration
        ../../gcc-trunk/gcc/cp/parser.c:12095
0x7874b6 cp_parser_declaration_seq_opt
        ../../gcc-trunk/gcc/cp/parser.c:12022
0x7877c4 cp_parser_translation_unit
        ../../gcc-trunk/gcc/cp/parser.c:4324
0x7877c4 c_parse_file()
        ../../gcc-trunk/gcc/cp/parser.c:37475
0x8e7e42 c_common_parse_file()
        ../../gcc-trunk/gcc/c-family/c-opts.c:1064
...