[Accepted as a DR at the February, 2019 meeting.]

Consider the following example:

   struct A {} a;
   struct B { explicit B(const A&); };

   struct D { D(); };
   struct C { explicit operator D(); } c;

   B b1(a);            // #1, ok 
   const B &b2{a};     // #2. ok 
   const B &b3(a);     // #3, error 

   D d1(c);            // ok 
   const D &d2{c};     // ok 
   const D &d3(c);     // #6, ok 

The disparity between #3 and #6 is suprising, as is the difference from #1 and #2. The reason for this difference is in 9.5.5 [dcl.init.list] bullet 3.10:

Otherwise, if T is a reference type, a prvalue of the type referenced by T is generated. The prvalue initializes its result object by copy-list-initialization or direct-list-initialization, depending on the kind of initialization for the reference. The prvalue is then used to direct-initialize the reference.

(reflecting the resolution of issue 1494).