[Bug c++/71548] Invalid declaration involving template template param causes crash

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71548

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ice-on-invalid-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-06-17
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to fail|                            |4.7.0, 5.3.0, 6.1.0, 7.0

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed as ice-on-invalid-code.  My bisection points to r179441 as the first
commit that triggered an ICE but since the commit isn't relevant to the C++
front end it must have been one before it and after r179435 which fails with:
sorry, unimplemented: cannot expand âT ...â into a fixed-length argument list
Looking at the commits in between r179436 is the only possible root cause.

Recent trunk fails with the following output:

$ cat t.C && gcc -Wall t.C
template<typename> class fl {};
template<typename = void, template<class...> class = fl>
struct S {};
template<typename... T>
void f(S<T...> ) {}
void lol() {
    S<> s;
    f(s);
}
t.C: In substitution of âtemplate<class ... T> void f(S<T ...>) [with T =
<missing>]â:
t.C:8:8:   required from here
t.C:8:8: internal compiler error: tree check: expected class âtypeâ, have
âdeclarationâ (template_decl) in cp_type_quals, at cp/typeck.c:9154
     f(s);
        ^
0x13ad713 tree_class_check_failed(tree_node const*, tree_code_class, char
const*, int, char const*)
        ../../gcc/tree.c:9803
0x72a08d tree_class_check(tree_node const*, tree_code_class, char const*, int,
char const*)
        ../../gcc/tree.h:3409
0x911f1e cp_type_quals(tree_node const*)
        ../../gcc/cp/typeck.c:9154
0x803b7c check_cv_quals_for_unify
        ../../gcc/cp/pt.c:19053
0x807fab unify
        ../../gcc/cp/pt.c:19707
0x8010f4 unify_one_argument
        ../../gcc/cp/pt.c:18292
0x8045b2 unify_pack_expansion
        ../../gcc/cp/pt.c:19203
0x808fac unify
        ../../gcc/cp/pt.c:19969
0x80382f try_class_unification
        ../../gcc/cp/pt.c:18965
0x8093af unify
        ../../gcc/cp/pt.c:19999
0x8010f4 unify_one_argument
        ../../gcc/cp/pt.c:18292
0x80139d type_unification_real
        ../../gcc/cp/pt.c:18366
0x7ff912 fn_type_unification(tree_node*, tree_node*, tree_node*, tree_node*
const*, unsigned int, tree_node*, unification_kind_t, int, bool, bool)
        ../../gcc/cp/pt.c:17811
0x73495c add_template_candidate_real
        ../../gcc/cp/call.c:3110
0x734db1 add_template_candidate
        ../../gcc/cp/call.c:3188
0x73c9ed add_candidates
        ../../gcc/cp/call.c:5361
0x737b34 perform_overload_resolution
        ../../gcc/cp/call.c:4045
0x737d9b build_new_function_call(tree_node*, vec<tree_node*, va_gc,
vl_embed>**, bool, int)
        ../../gcc/cp/call.c:4122
0x954894 finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool,
bool, int)
        ../../gcc/cp/semantics.c:2433
0x898a02 cp_parser_postfix_expression
        ../../gcc/cp/parser.c:6904
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