Must ranges support `r1 = r2;`?

[ag0aep6g]

Long version: <https://issues.dlang.org/show_bug.cgi?id=18657> 
("std.range and std.algorithm can't handle refRange").

Short version: With two `std.range.RefRange`s, `r1 = r2;` does not what 
other Phobos code expects.

Question is: Who's at fault? Where do I fix this? Do ranges have to 
support assignment as expected - even though std.range doesn't mention 
it? Or should range-handling code never do that - even though it comes 
naturally and is widespread currently?