[Bug c++/63999] New: Explcit conversion operators are not considered for explicit cast using brace

["tomaszkam at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63999

            Bug ID: 63999
           Summary: Explcit conversion operators are not considered for
                    explicit cast using brace
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomaszkam at gmail dot com

For the following code:
struct bool_convert
{
  explicit operator bool() { return true; }
};

bool x(bool_convert());        //1
bool x1{bool_convert{}};       //2
bool x2 = bool{bool_convert{}}; //3
Both line 1 and 2 compiles correcty, while the line 3 produces following error:
error: cannot convert 'bool_convert' to 'bool' in initialization.

According to the standard section $5.2.3 [expr.type.conv]/3:
Similarly, a simple-type-specifier or typename-specifier followed by a
braced-init-list creates a temporary
object of the specified type direct-list-initialized (8.5.4) with the specified
braced-init-list, and its value is
that temporary object as a prvalue.

That means that cast notation from the line 3 should have equivalent semantics
to the one from line 2.

Section $12.3.2 [class.conv.fct]/2:
A conversion function may be explicit (7.1.2), in which case it is only
considered as a user-defined conversion
for direct-initialization (8.5). Otherwise, user-defined conversions are not
restricted to use in assignments
and initializations.

Explicit conversion operator should be invoked for direct initialization so
behaviour for line 3 is incoorect and conversion operator should be used.