[Bug c++/78137] New: [C++1z] braced initializer in defaulted function argument causes ICE

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78137

            Bug ID: 78137
           Summary: [C++1z] braced initializer in defaulted function
                    argument causes ICE
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

With -std=gnu++1z, the following code ICEs the compiler:

```
struct S {};

template <class T = S>
void foo(T t = {}) {}

int main() {
  foo();
}
```

Result:

test.cpp: In function ‘int main()’:
test.cpp:6:7: internal compiler error: in build_over_call, at cp/call.c:7847
   foo();
       ^

The same code compiles successfully with -std=gnu++14.